Asymmetric chemical strengthening

ABSTRACT

Asymmetrically strengthened glass articles, methods for producing the same, and use of the articles in portable electronic device is disclosed. The asymmetrically strengthened glass articles include glass articles having a deeper compressive stress layer in a thicker portion of the glass article. Using a budgeted amount of compressive stress and tensile stress, asymmetric chemical strengthening is optimized for the utility of a glass article. In some aspects, the strengthened glass article can be designed for reduced damage, or damage propagation, when dropped.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional PatentApplication No. 62/645,789, filed Mar. 20, 2018 and titled “AsymmetricChemical Strengthening and this application is a continuation-in-partpatent application of U.S. patent application Ser. No. 15/600,204, filedMay 19, 2017 and titled “Asymmetric Chemical Strengthening,” whichclaims the benefit of U.S. Provisional Patent Application No.62/339,062, filed May 19, 2016 and titled “Asymmetric ChemicalStrengthening,” U.S. Provisional Patent Application No. 62/362,578,filed Jul. 14, 2016 and titled “Asymmetric Chemical Strengthening,” U.S.Provisional Patent Application No. 62/368,787, filed Jul. 29, 2016 andtitled “Asymmetric Chemical Strengthening,” and U.S. Provisional PatentApplication No. 62/368,792, filed Jul. 29, 2016 and titled “AsymmetricChemical Strengthening,” the disclosures of which are herebyincorporated herein by reference in their entireties.

FIELD

The described embodiments relate generally to asymmetric chemicalstrengthening of a glass article. More particularly, the presentembodiments relate to calibrating the strength and safety of a coverglass for use in a portable electronic device.

BACKGROUND

The cover window and display for small form factor devices are typicallymade of glass. Glass, although transparent and scratch resistant, isbrittle and prone to impact failure. Providing a reasonable level ofstrength in these glass parts is crucial to reducing the likelihood ofglass part failure, and hence device failure.

Chemical strengthening has been used to increase the strength of glassparts. Typical chemical strengthening relies on a uniform and symmetricincrease of the compression stress over the entire surface of the glasspart. Such strengthening processes have proven effective at reducingsome level of failure in glass parts. However, there continues to besignificant pressure on forming thinner glass for use in small formfactor devices, where symmetric chemical strengthening is insufficientto prevent impact failure in a reliable fashion.

As such, while conventional chemical strengthening is effective, thereis a continuing need to provide improved and alternative ways tostrengthen glass, particularly, thin glass.

SUMMARY

Various embodiments described herein encompass asymmetricallystrengthened glass articles. Asymmetrically strengthened glass articlescan have enhanced reliability and safety as compared to symmetricallystrengthened glass articles. In embodiments, an asymmetricallystrengthened glass article has a first zone with a first stress pattern,and a second zone with a second stress pattern. The first stress patternand second stress pattern differ from one another. The differences inthe first stress pattern and second stress pattern can result in astress imbalance in the asymmetrically strengthened glass article.

In aspects of the disclosure, the glass article is asymmetricallychemically strengthened through an ion exchange process. In embodiments,the ion exchange process introduces a compressive stress layer (i.e., aresidual compressive stress layer) along one or more surfaces of theglass article. Asymmetric chemical strengthening can occur when thecompressive stress layer differs along the surfaces of the glassarticle. For example, a depth of the compressive stress layer at a frontsurface of the glass article may be greater than a depth of thecompressive layer at a rear surface of the glass article. In this case,the front surface of the glass article may more durable and impactresistant than the bottom surface. In addition, although the inclusionof additional compressive stress on the front surface may tend to causean increase in the tensile stress within the glass article, thisincrease in tensile stress may be compensated for by the shallower depthof compression on the rear surface.

In additional aspects, the glass article includes a thicker portion anda thinner portion, each of which is strengthened differently. Inembodiments, the glass article includes a peripheral portion which isthicker than a central portion of the glass article. The greaterthickness of the peripheral region may allow a greater extent ofchemical strengthening in this region without creating undesirablelevels of tensile stress in the glass article.

In embodiments, a depth of the compressive stress layer is greater inthe thicker peripheral portion than in the thinner central portion. Thispattern of asymmetric chemical strengthening allows a surface of theperipheral region to be more resistant to impact than a surface of thecentral portion. In further embodiments, different surfaces of theperipheral portion and/or the central portion may be asymmetricallystrengthened. For example, the depth and/or the surface compressivestress may differ between the front and the rear surface of a portion ofthe glass article.

In additional embodiments, a glass article for an electronic devicecomprises a first portion having a first thickness and a second portionhaving a second thickness greater than the first thickness. A centralzone of the electronic device may define the first portion and thesecond portion. The electronic device may further comprise a peripheralzone contiguous with and at least partially surrounding the centralzone. The peripheral portion may have a third thickness greater than thefirst thickness. The third thickness may also be greater than the secondthickness.

As previously discussed, the glass article may be asymmetricallychemically strengthened. For example, the compressive stress layer maybe deeper at an exterior surface of the thicker first portion ascompared to an exterior surface of the thinner first portion. Further,the glass article may be asymmetrically chemically strengthened so thatthe compressive stress layer in a given portion of the glass article isdeeper at the exterior surface than at an interior surface.

For example, a thinner central zone may be asymmetrically chemicallystrengthened to include a first compressive stress region extending froma central exterior surface to a first depth; and a second compressivestress region extending from a central interior surface to a seconddepth. The second depth may be less than the first depth. As anadditional example, a thicker peripheral zone may be asymmetricallychemically strengthened to include a third compressive stress regionextending from a peripheral exterior surface to a third depth greaterthan the first depth; and a fourth compressive stress region extendingfrom the peripheral interior surface to a fourth depth less than thethird depth. The central zone may have a first thickness and theperipheral zone may have a second thickness and at least partiallysurround the central zone.

As an additional example, the glass article includes a thinner firstportion and a thicker second portion and the glass article may beasymmetrically chemically strengthened so that the compressive stresslayer is deeper at a front and/or a rear surface of the thicker secondportion as compared to a front surface of the thinner first portion. Theglass article may also be asymmetrically chemically strengthened so thatthe compressive stress layer of at least one of the first portion andthe second portion is deeper at the front surface as compared to therear surface. In embodiments, a central zone of the glass articledefines the first portion and the second portion.

As an example, the first portion has a first thickness and comprises afirst front surface and a first rear surface. The second portion has asecond thickness greater than the first thickness, is contiguous withthe first portion, and comprises a second front surface and a secondrear surface. The second portion may also comprise a first wall surfaceadjoining the first rear surface and the second rear surface.

The first portion further comprises a first compressive stress regionhaving a first depth along the first front surface; and a secondcompressive stress region having a second depth, less than the firstdepth, along the first rear surface. The second portion furthercomprises a third compressive stress region having a third depth alongthe second front surface; and a fourth compressive stress region havinga fourth depth along the second rear surface, at least one of the thirddepth and the fourth depth being greater than the first depth. Inembodiments, the first rear depth is about equal to the second reardepth.

In embodiments, a peripheral zone at least partially surrounds thecentral zone and the peripheral zone comprises a third front surface anda third rear surface. The peripheral zone may also comprise a secondwall surface adjoining the third rear surface and the first rearsurface. In addition, the peripheral zone may further comprise a thirdwall surface adjoining the third rear surface and the second rearsurface. The peripheral zone further comprises a fifth compressivestress region having a fifth depth along the third front surface; and asixth compressive stress region having a sixth depth along the thirdrear surface, at least one of the fifth depth and the sixth depth beinggreater than the first depth.

Various embodiments described herein also encompass an asymmetricallystrengthened cover glass for use with an electronic device, where thecover glass is designed to reduce or limit damage resulting from animpact, for example, a drop. In embodiments, the asymmetricallystrengthened cover glass includes

In additional embodiments, the cover glass includes three differentstress patterns resulting from asymmetric strengthening, a first stresspattern corresponding to corner zones of the cover glass, a secondstress pattern corresponding to straight edge(s) or straight perimeterzones of the cover glass, and a third stress pattern corresponding tothe remainder or center zone of the cover glass. The first zone has beenstrengthened the most, the second zone to a lesser extent than the firstzone, and the third zone the least, as compared to the first and secondzones. In order to maintain a stress budget that corresponds to a usefulcover glass for an electronic device, all of the stress budget istypically spent on the first and second zones, allowing little or nostrengthening of the third zone. This pattern of asymmetricstrengthening causes the corners, where most impacts occur, to be moststrengthened and resistant to impact, the second zone having adequatestrengthening for impact protection, and the third zone to remainsubstantially flat.

Embodiments also include portable electronic devices that include glassarticles in accordance with the disclosure, as well as to methods ofmanufacturing the same portable electronic devices. In some aspects, theglass articles can undergo monitoring and testing to identify conformingasymmetrically strengthened glass articles for use in electronicdevices.

In method embodiments, a glass article is asymmetrically strengthened tocalibrate the glass for use in a portable electronic device. The glassarticle can be calibrated to have a target geometry or provide one ormore flat surfaces.

In aspects, the disclosure provides a method for manufacturing a glassarticle, comprising forming a compressive stress layer through at leastone ion exchange along surfaces of the glass article. The compressivestress layer comprises regions of different depths in different portionsof the glass article. The glass article may comprise a thinner centralportion and a thicker peripheral portion and the compressive stresslayer may be formed along the central portion and the peripheralportion.

An example compressive stress layer comprises a first compressive stressregion having a first depth along a central exterior surface of theglass article and a second compressive stress region having a seconddepth along a central interior surface of the glass article. The seconddepth may be less than the first depth. The compressive stress layerfurther comprises a third compressive stress region having a thirddepth, greater than the first depth, along a peripheral exterior surfaceof the glass article, the peripheral portion having a thickness greaterthan a thickness of the central portion; and a fourth compressive stressregion having a fourth depth, less than the third depth, along aperipheral interior surface of the article. The formation of thecompressive stress layer produces a region of a tensile stress withinthe glass article to balance the compressive stress layer.

Typically multiple ion exchanges (alternately, ion exchange operations)are used to form a compressive stress layer including multiplecompressive stress regions. As an example, each of the compressivestress regions may be formed in a separate ion exchange operation. Asanother example, at least one compressive stress region may be formedduring multiple ion exchange operations. The operation of forming acompressive stress layer having regions of different depths generallyincludes at least one operation of applying a mask to the glass article.

Some methods of asymmetric strengthening include immersing an ionexchangeable glass in a bath comprising the ions to be exchanged forsmaller ions in the glass article. An example method may compriseimmersing a sodium-infused glass article in a potassium ion bath, whilepreferentially transporting the potassium ions at a predeterminedsurface of the glass article. In some aspects the immersing of thesodium-infused glass article in the potassium ion bath is accompanied bysubmitting microwave radiation to the same predetermined surface of theglass article. In methods including multiple ion exchange operations,the different baths may have different concentrations of the ions to beintroduced into the glass article.

In additional method embodiments, a stress relationship is identifiedand implemented using chemical strengthening. In some aspects, glassforming is combined with asymmetric chemical strengthening to provide aglass article having an appropriate geometry.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be readily understood by the following detaileddescription in conjunction with the accompanying drawings, wherein likereference numerals designate like structural elements, and in which:

FIG. 1 shows a diagram of a glass article in accordance with embodimentsherein.

FIG. 2 is a flow diagram of a glass strengthening process in accordancewith embodiments herein.

FIG. 3 shows a glass strengthening system in accordance with embodimentsherein.

FIG. 4A is a cross-sectional diagram of a glass cover which has beensymmetrically chemically treated in accordance with embodiments herein.

FIG. 4B is a cross-sectional diagram of a glass cover which has beensymmetrically chemically treated, as shown to include a chemicallytreated portion in which potassium ions have been implanted inaccordance with embodiments herein.

FIG. 5A is a diagram of a lattice structure for glass.

FIG. 5B is a diagram of a lattice structure for corresponding densifiedglass.

FIG. 6 is a diagram of a partial cross-sectional view of a glass cover,which shows two zones of densified glass.

FIG. 7A is a diagram of a partial cross-sectional view of a glass cover,which shows a tension/compression stress profile in accordance withembodiments herein.

FIG. 7B is a diagram of a partial cross-sectional view of a glass cover,which shows a reduced tension/compression stress profile in accordancewith embodiments herein.

FIG. 7C is a diagram of a partial cross-sectional view of a glass cover,which shows an asymmetric tension/compression stress profile inaccordance with embodiments herein.

FIG. 8 is a flow diagram of asymmetric glass strengthening in accordancewith embodiments herein.

FIG. 9 is a cross-sectional diagram of a glass cover which has beenasymmetrically chemically treated.

FIG. 10 is a cover glass having a silicon nitride coating applied to thecenter portion, while the edge and corner portions remain uncoated.

FIG. 11A is a cross-sectional diagram of a glass cover having acombination of coatings applied to the top and bottom surfaces.

FIG. 11B is a cross-sectional diagram of a glass cover that illustratesthe coating embodiments described in FIG. 11A.

FIGS. 12A and 12B illustrate the use of high ion concentration pastes onthe front and back surfaces of a cover glass.

FIG. 13 shows an alternative glass strengthening system in accordancewith embodiments herein.

FIGS. 14A-14E illustrates processing to chemically strengthen a pre-bentglass in accordance with embodiments herein.

FIG. 15 shows a glass strengthening system for clad layered glassarticles in accordance with embodiments herein.

FIG. 16 is a flow diagram of glass article production using asymmetricglass treatment.

FIGS. 17A and 17B illustrate chemically strengthening at potentialfracture spots to minimize fracture propagation.

FIG. 18 is a fracture pattern stress plot in accordance with embodimentsherein.

FIG. 19 is a flow diagram of glass article production where the glassarticle has at least three zones of different chemical strengthening.

FIG. 20 is a flow diagram of cover glass production where the glassarticle has the greatest amount of chemical strengthening in itscorners, a lesser amount of chemical strengthening along its perimeterside edges and the least amount in the remainder of the glass.

FIG. 21 shows a diagram of a cover glass in accordance with embodimentsherein.

FIG. 22 shows a cross-sectional view of the corner in FIG. 19 toillustrate asymmetric chemical strengthening.

FIG. 23 is a flow diagram for compensating for asymmetric chemicalstrengthening with glass forming techniques in accordance withembodiments herein.

FIG. 24 illustrates a stress profile for an asymmetrically strengthenedcover glass.

FIG. 25 illustrates a glass article formed to a predetermined geometryin accordance with embodiments herein.

FIG. 26 illustrates a glass article, after forming, subjected to CNC andpolishing in accordance with embodiments herein.

FIG. 27 illustrates the glass article, after forming and CNC, locallycoated with a diffusion barrier (SiN) in accordance with embodimentsherein.

FIGS. 28A and 28B illustrate asymmetric chemical strengthening of theglass article of FIG. 12 in accordance with embodiments herein.

FIG. 28C is a stress profile in accordance with the glass article shownin FIG. 23A.

FIGS. 29A and 29B illustrate oxidation of the silicon nitride layer on aglass article to SiO₂ in accordance with embodiments herein.

FIGS. 30A and 30B illustrate asymmetric chemical strengthening to aformed glass article in accordance with embodiments herein.

FIG. 30C is a stress profile in accordance with the glass article shownin FIG. 30A.

FIG. 31 is a top view of a glass article having a central zone and aperipheral zone.

FIG. 32A is a simplified cross-section view of a glass article having acentral portion which is thinner than a peripheral portion.

FIGS. 32B and 32C illustrate examples of compressive stress regionsformed in the central portion and the peripheral portion of the glassarticle of FIG. 32A.

FIG. 33A is a simplified cross-section view of another glass articlehaving a central portion which is thinner than a peripheral portion.

FIG. 33B illustrates an example of compressive stress regions formed inthe central portion and the peripheral portion of the glass article ofFIG. 33A.

FIGS. 34A and 34B show views of a second sample glass article havingportions of different thickness and an asymmetrically chemicallystrengthened layer that changes depth.

The use of cross-hatching or shading in the accompanying figures isgenerally provided to clarify the boundaries between adjacent elementsand also to facilitate legibility of the figures. Accordingly, neitherthe presence nor the absence of cross-hatching or shading conveys orindicates any preference or requirement for particular materials,material properties, element proportions, element dimensions,commonalities of similarly illustrated elements, or any othercharacteristic, attribute, or property for any element illustrated inthe accompanying figures.

Additionally, it should be understood that the proportions anddimensions (either relative or absolute) of the various features andelements (and collections and groupings thereof) and the boundaries,separations, and positional relationships presented therebetween, areprovided in the accompanying figures merely to facilitate anunderstanding of the various embodiments described herein and,accordingly, may not necessarily be presented or illustrated to scale,and are not intended to indicate any preference or requirement for anillustrated embodiment to the exclusion of embodiments described withreference thereto.

DETAILED DESCRIPTION

Reference will now be made in detail to representative embodimentsillustrated in the accompanying drawings. It should be understood thatthe following descriptions are not intended to limit the embodiments toone preferred embodiment. To the contrary, they are intended to coveralternatives, modifications, and equivalents as can be included withinthe spirit and scope of the described embodiments as defined by theappended claims.

The following disclosure relates to glass articles, methods of producingglass articles, and to the utility of such glass articles in anelectronic device. The glass article may be a glass component of anelectronic device. Embodiments also relate to the asymmetric increase inthe strength of glass, especially related to asymmetricallystrengthening a glass article to further calibrate the reliability andsafety of the glass article in an electronic device. In some embodimentsthe electronic device can include a housing, a display positioned atleast partially within the housing and a glass article, for example acover glass, in accordance with embodiments herein.

In one example, the glass article may be an outer surface of anelectronic device. The glass article may correspond to a glass articlethat helps form part of a display area or, in some instances, beinvolved in forming part of the housing. The embodiments herein areparticularly relevant for use in portable electronic devices and smallform factor electronic devices, e.g., laptops, mobile phones, mediaplayers, remote control units, and the like. Typical glass articlesherein are thin, and typically less than 5 mm in thickness, and in mostcases are between about 0.3 and 3 mm, and between 0.3 and 2.5 mm, inthickness.

These and other embodiments are discussed below with reference to FIGS.1-34B. However, those skilled in the art will readily appreciate thatthe detailed description given herein with respect to these figures isfor explanatory purposes only and should not be construed as limiting.

FIG. 1 is a perspective diagram of a glass article in accordance withone embodiment. The glass article 100 is a thin sheet of glass with alength and width consistent with the application. In one application asshown in FIG. 1, the glass article is a cover glass for a housing of anelectronic device 103. In embodiments, the various surfaces of the glassarticle may be referenced with respect to their orientation in anelectronic device. For example, the glass article may have a surfacewhich faces an exterior of the electronic device. This surface may alsoform an external surface of the electronic device. This surface may bereferred to as an exterior surface or as an outer surface. The exteriorsurface may include a front surface of glass article. Similarly, theglass article may have a surface which faces an interior of theelectronic device. This surface may be referred to as an interiorsurface or an inner surface. The interior surface may include a back orrear surface of the glass article. The terms “interior,” “exterior,”“front”, and “rear” are used to identify surfaces of the glass articlerelative to the electronic device; the orientation of the apparatus isnot intended to limited by the use of these terms. Some glass articlesmay also include at least one side surface between the interior surfaceand the exterior surface. A periphery of the glass article may bedefined at least in part by the at least one side surface.

As illustrated in FIG. 1, the glass article 100 can have a front surface102, back surface (not shown), top surface 104, bottom surface 106, andside surfaces 108, and edges 110. Alternately, the top surface 104 andthe bottom surface 106 may simply be referred to as side surfaces. Asshown, an edge 110 may provide a transition between surfaces at theperiphery of the glass article (e.g. between surface 106 and surface108). As discussed in more detail below, the edges 110 of the glassarticle 100 can have predetermined geometries. The various surfaces andsides can be composed of zones and/or portions. For example, a glassarticle may comprise a peripheral zone and a central zone. Theperipheral zone (or peripheral portion) may include a peripheral regionof the external surface and a peripheral region of the internal surface.The peripheral zone may further include the side surfaces of the glassarticle. The peripheral zone may form a boundary around at least aportion of the central zone (also central portion or center portion).

As another example, one zone of a glass article could be the entirefront surface, while the back surface would be considered a differentzone, for example. Another zone of a glass article could be an areacorresponding to one or more corners of the glass. A zone does not haveto be continuous, for example all four corners of the glass article maybe representative on a single zone. The strength requirements for thesurfaces and zones may differ on the use, for example, a front surface102, exposed to the outside environment, may require a differentstrength than the back surface, enclosed away from the environment.

These and other embodiments are discussed below with reference to FIGS.2-30. However, those skilled in the art will readily appreciate that thedetailed description given herein with respect to these Figures is forexplanatory purposes only and should not be construed as limiting.

Chemical Strengthening

Embodiments herein may utilize a glass strengthening process where aglass article is first enhanced by immersion in a first ion solution(sodium, for example) and then strengthened by immersion in a second ionsolution (potassium, for example).

FIG. 2 is a flow diagram of a glass strengthening process 200 accordingto one embodiment. The glass strengthening process 200 includesobtaining a piece of glass 202, enhancing the glass article throughchemical processing 204, and strengthening the glass article throughfurther chemical processing 206.

FIG. 3 illustrates one embodiment for strengthening a glass article inaccordance with embodiments herein 300. A glass article 302 in need ofglass strengthening is immersed in a first bath 304 that contains asodium solution 306. The enhanced strength glass article is then removedfrom the first bath 304 and immersed in a second bath 308 that containsa potassium solution 310. At this stage, the glass article 302 issymmetrically strengthened, meaning that all exposed surfaces of theglass article have been equally enhanced and strengthened through theimmersion in the sodium and then potassium solutions. In someembodiments, the strengthened glass article can be quenched to eliminatefurther exchange of ions from the treated glass article.

The level of glass article enhancement is generally controlled by thetype of glass (glass articles can, for example, be alumina silicateglass or soda lime glass, and the like); the sodium concentration of thebath (sodium or sodium nitrate, typically 30%-100% mol); the time theglass article spends in the bath (typically 4-8 hours); and temperatureof the bath (350-450° C.).

Strengthening of the glass article in the second bath is controlled bythe type of glass, the potassium ion concentration, the time the glassspends in the solution, and the temperature of the solution. Here, thepotassium or potassium nitrate is in the range of 30-100% mol, but theglass article would remain in the bath for about 6-20 hours at asolution temperature of between about 300-500° C.

The chemical strengthening process relies upon ion exchange. In eachsolution bath the ions therein are heated to facilitate ion exchangewith the glass article. During a typical ion exchange, a diffusionexchange occurs between the glass article and the ion bath. For example,sodium ions in the enhancement process diffuse into the surface of theexposed glass, allowing a build-up of sodium ions in the surface of theglass by replacement of other ions found in a silicate or soda limeglass. Upon immersion of the enhanced glass article into the potassiumbath, the sodium ions are replaced by potassium ions in surface areas toa greater extent than sodium ions found more toward the interior ormiddle of the glass. As a result, the potassium ions introduced into theglass to replace the sodium ions form a compression layer near thesurface of the glass article (essentially the larger potassium ions takeup more space than the exchanged smaller sodium ions). The sodium ionsthat have been displaced from the surface of the glass article becomepart of the potassium bath ion solution. Depending on the factorsalready discussed above, a compression layer as deep as about 10-100microns, and more typically 10-75 microns, can be formed in the glassarticle. The surface compressive stress (CS) may be from about 300 MPato about 1100 MPa.

FIG. 4A is a cross-sectional diagram of a glass article 400 which hasbeen chemically treated such that a symmetrical chemically strengthenedlayer 402 is created according to embodiments described herein. Theglass article 400 includes a chemically strengthened layer 402 and anon-chemically strengthened inner portion 404. While discussed ingreater detail throughout, the effect of chemically strengthening theglass article is that the inner portion 404 is under tension, while thechemically strengthened layer 402 is in compression. The chemicallystrengthened layer has a thickness (Y) which may vary depending upon therequirements of a particular use.

FIG. 4B is a diagrammatic representation of a chemically strengthenedprocess. Note that some amount of sodium 405 diffuses from the enhancedglass article to the ion bath, while potassium (K) ions 406 diffuse intothe surface of the glass article, forming the chemically strengthenedlayer 402. Alkali metal ions like potassium, however, are generally toolarge to diffused into the center portion of the glass, thereby leavingthe interior portion 404 only under tension and not in compression. Bycontrolling the duration of the treatments, temperature of thetreatments, and the concentration of the various ions involved in thetreatments, the thickness (Y) of a strengthening compression layer 402may be controlled, as well as the concentration of ions in thecompression layer. Note that the concentration of the ions involved inthe chemically strengthening process may be controlled by maintaining,during glass article treatment, a substantially constant amount of ionin each of the two baths (for example, as the potassium ions diffuseinto the glass, a controller would add more potassium ions into the ionbath—thereby encouraging the potassium to continue to diffuse into theglass). The relationship between the chemically strengthened compressionlevel (both ion concentration at the surface and depth) and innertension portion forms a stress pattern for a chemically treated glassarticle.

Additional ion bath immersions may be added to the basic glass chemicalstrengthening process. For example, a third bath including sodium orsodium nitrate can be used to immerse the strengthened glass so as toexchange potassium ions out of the compression layer for sodium ions inthe third bath. This is referred to as a back-exchange or tougheningprocess. The toughening process is used to further control the depth andstrength of a compression layer, and in particular, to remove somecompression stresses from near the top surface regions, while allowingthe underlying potassium ions to remain in the lower regions of thecompression layer. In addition, the toughening process reduces thecentral tension from the glass article (see below).

Although sodium enhancement and potassium strengthening is describedherein, other ion combinations are within the scope of the presentdisclosure, for example, use of lithium instead of sodium, or cesiuminstead of potassium, e.g., sodium-potassium, sodium-cesium, lithium-potassium, lithium-cesium treatment combinations. Any ion combinationcan be used herein that provides an increase in the glass articlesurface compression and compression depth.

Chemical strengthening is applied to glass surfaces, and relies uponexposure of the glass surface to the chemical strengthening process.Where a glass article is immersed such that all aspects of the articlehave equal exposure to the ion bath, the glass article surface will besymmetrically strengthened, allowing for a glass article with auniformly thick and composed compression layer (Y). As embodimentsherein will show, where a glass article surface is not equally exposedto chemical strengthening, the surface will be asymmetricallystrengthened, allowing for a glass article with a non-uniformcompression layer. As above, asymmetrically strengthened glass articleshave a stress pattern; however, the stress pattern is modified based onthe asymmetry of the chemical treatment.

Pre-Heating to Increase Glass Density Prior to Chemical Strengthening

Chemical strengthening may be enhanced or facilitated by various thermaltechniques that are performed prior to the chemical strengtheningprocess. Chemical strengthening is limited by the saturation limit ofthe glass for an amount or volume of ions. The size, depth andconcentration of ions within a glass article directly relates to thecharacteristic strengthening for that glass, which as described herein,can be modified and calibrated throughout the glass to optimize theglass for a particular use.

At saturation, no additional compression layer or depth modificationsmay be accomplished (via diffusion). However, modification of thermalinput to a glass article, prior to chemical strengthening, can allow forenhancement of the glass surface density, which will directly contributeto the concentration and depth of the strengthened compression layer.

Where a significant amount of thermal energy is added to a glass articleprior to chemical strengthening, the glass density of the article can beincreased. Glass density in these embodiments results in the glasslattice being heated to a point of densification.

As shown in FIG. 5A and FIG. 5B, denser glass (5B) 500 provides a morelimited lattice structure (more restricted and less flexible) and isless able to undergo ion diffusion to deeper levels than non-treatedglass (5A) 502.

In FIGS. 5A and 5B, the glass has a starting glass lattice structure502, which when heated to a densification temperature, is densified andprovides a smaller volume 506 for ions to move through than the volume508 of the non-densified glass 502. In an embodiment, the latticestructure is a network structure, such as a silicate-based networkstructure. For example, an aluminosilicate glass may have analuminosilicate network structure. The restriction on the glass latticeallows for fewer ions to diffuse inwardly, while the concentration ofions in the chemical strengthening bath remains high (as compared to anion bath used for non-densified glass). Also, although the glass latticehas been densified, embodiments herein do not result in thermal input tothe point of crystal lattice collapse (not shown), rather heat isapplied to the point of lattice limitation, some ions are able todiffuse into the glass. The ions that do diffuse into the glass aretightly packed at the surface of the densified glass and thereby providea superior surface compression layer of shallow depth.

As such, the increase in glass density at the start of the chemicalstrengthening process limits ion diffusion into the glass surface,allowing the glass to exchange a greater amount of ions at the surfaceof the glass, but only allowing the exchange to a shallow depth. Glassarticles treated prior to chemical strengthening by initial thermalinput typically express a higher chemical stress at the surface, but toa shallower depth. These glass articles are most useful for highcompressive stress but to a shallow depth, e.g., an article wherepolishing or other like procedure is likely required on the chemicallystrengthened glass, or where the glass may be exposed to increased riskof scratching but not wear and tear (impact).

One such thermal technique is annealing a glass article prior tochemical strengthening. Annealing includes subjecting the glass articleto a relatively high temperature in an annealing environment for apredetermined amount of time, and then subjecting the glass article to acontrolled cooling for a second predetermined amount of time. Onceannealed and chemically strengthened, the glass article will have amodified compressive stress as compared to similar glass articles notannealed prior to chemical strengthening. As noted above, annealing isparticularly important where the glass article is in need of highsurface compressive stress (but to a shallower depth).

The annealing process requires that the glass article be heated to atemperature between the glass's strain point temperature and softeningtemperature, also known as the glass's annealing temperature (foraluminosilicate glass the annealing temperature is between about540-550° C.). The time required to anneal a glass article varies, but istypically between 1-4 hours, and cooling times typically are on theorder of ½° C./min for up to about 5 hours.

Typically, glass articles that have been annealed may be taken straightfrom a controlled cooling, and immersed in the enhancement ion bath(sodium), or, alternatively, the article may be further air cooled, andthen immersed in the first ion bath. Once annealed, the glass willresist deeper ion diffusion but allow some diffusion at the surface. Thediffusion into the surface allows for high compression stress (withshallow depth).

A second thermal technique used to raise a glass article's density priorto chemical strengthening is hot isostatic pressing or HIP. HIP includessimultaneously subjecting the glass article to heat and pressure for apredetermined amount of time in an inert gas. The glass article isallowed to remain in the HIP pressure vessel until the glass article isdenser, where internal voids in the glass are limited. As for annealing,the increase in glass density prior to chemical strengthening by HIPallows for the production of a higher compression stress at the glassarticle surface, but to a shallower depth (than would be expected for aglass article that does not undergo HIP).

HIP parameters vary, but an illustrative process would involve placingthe glass article to be chemically strengthened in a HIP pressurevessel, drawing a vacuum on the vessel, and applying heat to the glassarticle in the vessel. Under pressure, the vessel may be heated to600-1,450° C., depending on the type and thickness of the glass. Heatand pressure are typically maintained for about 10-20 minutes, afterwhich the processed glass is allowed to cool. In some embodiments, asuitable inert gas can be introduced in the vessel to facilitate heatingof the glass article. HIP is another tool for modifying or enhancing thechemical strengthening process.

As shown in FIG. 6, the pre-heating of the glass article 600 can belocalized (and not across the entire surface(s) of the glass article),such that target or predetermined zones 602 of the glass article aredensified. In this embodiment, localized heating (shown as arrows 604)is performed prior to chemical strengthening and to a point between theglass's strain point temperature and softening temperature. Laser orinductive coil heating can be used to pre-heat the location and therebyprovide a glass article that includes both densified 608 andnon-densified glass surfaces 610. FIG. 6 shows a simple cross section ofa glass cover 600 where the sides have been locally pre-heated to formdensified glass 608, while the center of the glass article exhibitsnon-densified glass 610.

Embodiments herein include glass articles pre-treated by heatingtechniques to form densified glass over an entire surface, or inpredetermined zones or locales, leaving zones of different glassdensity. When a glass article so treated is chemically strengthened 612,the article will be asymmetrically strengthened and have an asymmetricstress pattern, where densified glass exhibits a higher surfacecompression stress, but to a shallower depth, than correspondingnon-densified glass. It is envisioned that the timing and placement ofthe pre-heating can be used to optimize a glass surface compressivestress and the depth of the compressive stress.

Although not explicitly noted in all embodiments herein, all glassarticle embodiments herein may include the use of glass articles thathave been pre-heated to densify the glass prior to chemicalstrengthening.

Chemical Strengthening of Preferred Edge Geometries

Certain glass article edge geometries can also be used to strengthen aglass article for a particular utility in combination with chemicalstrengthening. For example, embodiments herein provide predeterminedgeometries useful in the strengthening of glass covers. Edgemanipulation can be accomplished, for example, by machining, grinding,cutting, etching, molding or polishing.

Illustrative rounded edge geometries for a glass cover useful in anelectronic device include manipulation of an edge to an edge radius of10% of the thickness of the cover glass, e.g., 0.1 mm edge radius for a1.0 mm thick glass cover. In other embodiments, the manipulation to theedge can include an edge radius of 20%-50% of the thickness of the coverglass, for example, 0.2 mm edge radius for a 1.0 mm thick glass cover,0.3 mm edge radius for a 1.0 mm edge radius, etc.

In general, some embodiments herein show that rounding of the edges of aglass cover increases the strength of the glass cover. For example,rounding an otherwise sharp edge on a glass cover improves the strengthof the edges, which thereby strengthens the glass cover itself. Ingeneral, the larger the edge radius, the more uniform the strengtheningcan be over the surface of the glass cover.

As such, in some embodiments herein, useful edge geometry can becombined with chemical strengthening to produce a more reliable anddurable glass cover. For example, chemically strengthening to increasethe compressive stress layer depth along the perimeter of a glass cover,combined with the four edges of the glass cover having an edge radius of30%.

Although not explicitly noted in all embodiments herein, all chemicallystrengthened glass article embodiments herein may include 1, 2, 3 or 4of its edges machined to a useful geometry. For cover glass designs therounding may be from 10-50% of the thickness of the cover glass.

Stress Profiles

Chemically treating a glass article in accordance with embodimentsherein effectively strengthens the exposed or treated surfaces of theglass. Through such strengthening, glass articles can be made strongerand tougher so that thinner glass can be used in portable electronicdevices.

FIG. 7A is a diagram of a partial cross-sectional view of a glassarticle, for example a glass cover. The diagram shows an initialtension/compression stress profile according to one embodiment. Theinitial tension/compression stress profile may result from an initialexchange process to symmetrically strengthen the surface region of theglass. A minus sigma legend indicates a profile region of tension, whilea plus sigma legend indicates a profile region of compression. Thevertical line (sigma is zero) designates crossover between compressionand tension.

In FIG. 7A, thickness (T) of the glass cover is shown. The compressivesurface stress (CS) (i.e., surface compressive stress) of the initialtension/compression stress profile is shown at the surface of the coverglass. The compressive stress for the cover glass has a compressivestress layer depth (DoL) that extends from surfaces of the glass covertowards a central region. Initial central tension (CT) of the initialtension/compression stress profile is at the central region of the glasscover.

As shown in FIG. 7A, the initial compressive stress has a profile withpeaks at the surfaces 700 of the glass cover 702. That is, the initialcompressive stress 704 is at its peak at the surface of the glass cover.The initial compressive stress profile shows decreasing compressivestress as the compression stress layer depth extends from surfaces ofthe glass cover towards the central region of the glass cover. Theinitial compressive stress continues to decrease going inwards untilcrossover 706 between compression and tension occurs. In FIG. 7A,regions of the decreasing profile of the initial compressive stress arehighlighted using right-to-left diagonal hatching.

The peaks at the surface of the glass cover provides an indication ofthe bending stress a glass article can absorb prior to failure, whilethe depth of the compressive layer provides protection against impact.

After crossover between compression and tension, a profile of theinitial central tension 708 extends into the central region shown in thecross-sectional view of the glass cover. In the diagram, FIG. 7A,regions of the decreasing profile of the initial central tension (CT)extending into the central region is highlighted using hatching.

Typically the combination of stresses on a glass article are budgeted toavoid failure and maintain safety, i.e., if you put too much stress intoa glass article, the energy will eventually cause the article to breakor fracture. Therefore, each glass article has a stress budget, anamount of compressive versus tensile strength that provides a safe andreliable glass article.

FIG. 7B is a diagram of a partial cross-sectional view of a glass cover,which shows a reduced tension/compression stress profile according toone embodiment. The reduced tension/compression stress profile mayresult from a double exchange process. Reduced compressive surfacestress (CS′) of the reduced tension/compression stress profile is shownin FIG. 7B. The compressive stress layer depth (D) now corresponds tothe reduced compressive stress. In addition, reduced central tension(CS′) is shown in the central region.

In light of FIG. 7B, it should be understood that the reducedcompressive surface stress (CS′) shows increasing profiles as thecompressive surface layer depth extends from surfaces of the glass coverand towards the submerged profile peaks. Such increasing profiles ofcompressive stress may be advantageous in arresting cracks. Within adepth (DoL) of the submerged peaks, as a crack attempts to propagatefrom the surface, deeper into the cover glass, it is met with increasingcompressive stress (up to DP), which may provide crack arresting action.Additionally, extending from the submerged profile peaks further inwardtoward the central region, the reduced compressive stress turns toprovide a decreasing profile until crossover between compression andtension occurs.

FIGS. 7A and 7B show a symmetric stress profile, where both sides of thecover glass have equal compressive stress, compressive stress layerdepth, and central tension.

FIG. 7C shows an asymmetric stress profile for a glass article 714 wherethe top surface 716 shows a more significant compressive stress CS andcompressive stress layer depth (DoL) than the bottom surface 718. Notethat the top surface 716 would, in this case, be more durable and impactresistant than the bottom surface. Also note that there is a stressbudget, the inclusion of additional compressive stress on the surfacemay be compensated for by a much shallower depth of compression on thebottom surface. In the absence of the compensation, the tensile force720 would be extended to the left and ultimately result in a highlyunsafe glass cover (tensile strength would overcome compressivestrength).

As will be discussed in greater detail below, design and production ofglass cover articles having modified stress profiles like FIG. 7C forcalibrated utility, are accomplished by using the asymmetric chemicalstrengthening processes described herein. By asymmetricallystrengthening a glass article, calibrated and highly useful glassarticles may be produced. In such instances, the stress budget for anypiece of glass may be used to provide a stress profile, and thereforeglass article, having an optimized surface for its utility.

Asymmetric Chemical Strengthening

Embodiments herein result in the production of asymmetricallystrengthened glass articles. Asymmetrically strengthened glass articles,for example cover glass, can be designed to be more reliable, damageresistant, and safer than corresponding symmetrically strengthened glassarticles.

FIG. 8 shows an illustrative flow diagram for asymmetricallystrengthening a glass article 800. A glass article is identified for adesired utility based on its dimensions, its thickness, and its inherentcomposition 802. A budget for how much stress the identified glass canwithstand is determined based on the glass's utility 804, and a budgetdetermined for optimal reliability and safety for the glass, i.e., thestress in the glass is balanced to provide both strength and safety 806.The glass article is then calibrated to exhibit a useful stress patternso as to maximize the stress budget and utility through use ofasymmetric chemical strengthening 808.

For example, a piece of thin cover glass used on a portable electronicdevice optimally requires different properties over its surface.Asymmetry of the chemical strengthening may be required on the front-versus the back-side of a glass article, on the perimeter versus thecenter of a glass article, around features in a glass article, or inhard to polish areas in a glass article. However, as discussed above,each glass article has a stress pattern to avoid failure, where thecompressive stress and tensile stress must be roughly balanced. As such,asymmetric chemical strengthening is used to optimize the properties ofa particular glass article, within the glass article's stress budget,for a particular use.

In general, asymmetric chemical strengthening can be used to provide ahigher (or lower) surface compression layer or a deeper (or shallower)stress layer, for a particular region, while maintaining the safety ofthe glass by not overstressing the tensile stress within the glassarticle. Where a surface of glass requires additional strength, thecompression of the layer may be increased, where the glass requiresprotection from wear and tear, the depth of the compression layer may bemodified, and the like. The ability to maximize the stress within aglass article for a zone or portion of a glass article, allows for thedesign of reliable and safe glass parts. In general, the relationship ofthe compressive stress (amount and depth) on the top and bottom surfaceof a glass article in relationship to the resultant tensile stress givesa stress pattern for the glass article. The stress pattern can be alongthe X, Y or Z axis of the glass article.

In embodiments herein, asymmetric chemical strengthening of a glassarticle is provided to: increase the reliability of a glass article fora particular use; to increase the safety of a glass article for aparticular use; to facilitate target shapes or forms (flat orsubstantially flat) of a glass article for a particular use; to be usedin combination with other techniques to facilitate a glass article'starget shape or form; and other like utilities.

FIG. 9 shows that asymmetric chemical strengthening is dependent ondifferentially incorporating ions into a surface of a glass article. Asnoted above, a glass article 900, along any surface area 902, canexchange and incorporate ions to a particular depth and concentrationbased on the glass articles' density and overall ion saturation point,i.e., there is only so much volume in the glass that can be involved inexchange to larger sized ions, so to increase the articles compression(see 901 versus 903). The change in ion concentration along the surface,and to particular depths, modifies the glass internal stressrelationship, this relationship extends across the thickness of theglass 904, as well as throughout the glasses interior portion (how theinternal tension/compression stress changes across the middle of theglass article) 906. As such, and as discussed previously, a stresspattern can be across the thickness of a glass article (vertical—top tobottom surface) 904 as well as across or throughout the glass article(horizontal—side to side) 906.

Embodiments herein utilize these stress relationships to calibratedutilities to provide modified glass articles for use in portableelectronic devices and small form factor devices.

Asymmetric Strengthening via Masking or Coating

Embodiments herein include the application of masking or ion-diffusionbarriers to portions of a glass article prior to immersion in the ioncontaining baths. For example, a portion of the glass surface can bephysically masked from the ions in the chemical strengthening processvia a diffusion impermeable material, such as a metal or ceramic, sealedover the region where diffusion is not wanted. This type of physicalmasking completely limits ion-diffusion into that surface and providesasymmetric strengthening, i.e., the masked surface will receive no ionexchange as compared to the other exposed surfaces of the glass article.Once chemically treated, the physical barrier would typically be removedfrom the glass article. Here you would have treated and untreatedsurfaces.

In another embodiment, as shown in FIG. 10, a coating or film composedof silicon nitrate (e.g., SiN, Si₃N₄), or other like material, is usedinstead of a physical mask. In FIG. 10, a coating 1000 is applied to thecentral portion of a glass cover 1002, while the edges and corners 1004are left uncoated. Such a coating would limit or eliminate ion diffusionat the center zone or portion of the cover glass, while allowingchemical strengthening at the non-coated zones (edges and corners).

The coating is first applied to the glass article prior to theenhancement treatment to block substantially all ion diffusion throughthe coated portion of the glass article. Coatings can have a thicknessof from about 5-500 nm, although other thicknesses may be used whereappropriate. In this illustration, the coated surface of the glassarticle, upon completion of the chemical strengthening process, wouldnot include a compression layer, whereas the remainder of the glassarticle would exhibit a compression layer. Upon completion of thechemical strengthening process, the coating could be removed viapolishing from the glass article, providing a surface having asymmetricstrengthening, or could be left on the surface of the glass, as part ofthe finished glass article. In this aspect, the coating would betailored to an appropriate thickness and composition in order to remainpart of the glass article.

In other embodiments, the silicon nitride coating can be oxidized afterthe chemical strengthening process is complete to provide a moreion-permeable barrier. The same glass article may now be re-immersed andprocessed through chemical strengthening, such that some ion diffusionoccurs through the silicon dioxide barrier, and thereby some compressionlayer is formed at the locale (while the remainder of the glass articlehas been treated twice).

As just noted, a coating composed of alternative materials, silicondioxide for example, can also be used to limit, rather than eliminate,ion diffusion to the surface of the glass article. For example, acoating composed of silicon dioxide would only limit ion diffusion tothe glass article surface, allowing some level of compression layerformation in the coated region, but not the complete strengtheningcontemplated by the ion exchange baths. As above, the coating would beeither removed upon completion of the chemical strengthening process, orleft in place as part of the finalized article. In either case, theglass article would have a surface with asymmetric strengthening.

FIG. 11 shows combinations of coating types (1100, 1102, 1104 . . . )and thicknesses can be used in designing an asymmetrically strengthenedglass surface. In FIG. 11A, a series of coatings (1100, 1102, 1104) areapplied to both the top and bottom surface (1106 and 1108, respectively)of a glass cover 1110. Each combination of coating material is meant tocontrol ion diffusion to the target glass surface, and thereby modifythe chemical strengthening of that surface 1112.

The glass article can exchange and incorporate ions to a particulardepth and concentration based on the ion diffusion through coatings1100, 1102 and 1104. As described previously, the change in ionconcentration along the surface, and to particular depths, modifies theglass internal stress relationship. The stress pattern shown in FIG. 11Billustrates that the edges 1114 of the top surface 1106, having nocoating, receives the most robust ion concentration along the surface,and to the greatest depth. The remainder of the top surface 1106 showssome reduced ion incorporation, but to a lower extent than at the edges1116. The bottom surface 1108, being internal, for example, has multiplezones defining three areas of ion incorporation 1116, 1118, 1120, basedon the layered coatings. The center zone 1120 of the bottom surface haslittle or no ion incorporation due to coatings 1100, 1102, and 1104. Thecombined coatings eliminate almost all ion diffusion into the centerzone. The other zones show some ion diffusion that results from eitherthe single coating or the combination coating. Thus, a stressrelationship where multiple coatings (ion barriers) have been applied toprepare an asymmetrically strengthened glass article is achieved.

It is further envisioned that multiple layers of coating can also beused to control the ion diffusion process into the target glass surface.For example, a thin coating that limits sodium and potassium iondiffusion from a chemical strengthening process by 25%, could be layeredacross a first thicker coating that limits sodium and potassium iondiffusion by 50%. The glass surface region would potentially have aregion limited of ion diffusion by 0% (uncoated), 25% (first coat), 50%(second coat), and 75% (layered coat); other embodiments may havedifferent percentages for each coat. As above, the finished glassarticle surface could include each of the coating layers, or could betreated to remove the coatings, leaving only the underlyingasymmetrically strengthening surfaces. It is also envisioned that theion-diffusion barrier coatings can be combined with the ion-barriermasks to further allow for calibrated glass article surfacestrengths—for example, physically mask a bottom surface of the glasscover and coat patterns or locales with a 25% ion diffusion barrier onthe top surface of the cover.

Thermal Assisted Asymmetric Chemical Strengthening

Embodiments herein include asymmetric glass strengthening during thechemical strengthening process through the targeted application of heat.Preferential heating of a glass surface locale can be used to facilitatestress relaxation in that locate, and thereby allow for an increase inion diffusion at that locale during the chemical strengthening process.Note that the heat is below the amount required to densify the glass asdiscussed above. An increase in ion diffusion allows for the exchange ofadditional ions into the glass, thereby changing the stress profile forthe heated surface compared to the non-heated surface. For example, alocalized region of a glass article can be heated through the use ofheating coils, laser, microwave radiation, and the like, while the glassarticle is immersed in a chemical strengthening ion bath.

As noted above, the increase in heat at the target locale allows for anincrease in ion diffusion in the glass surface at the heated locale.Enhanced heating of target locales on the glass surface providesasymmetric chemical strengthening at the heated locales as compared tonon-heated surfaces. Asymmetric chemical strengthening using modifiedthermal profiles is of particular value where a laser or microwave beamcan be directed to modify the chemical strengthening for parts havingknown failure spots. For example, cover glass that requires additionalchemical strengthening at the corners to limit breakage as a result ofimpact.

Heating temperatures are appropriate where the heat is sufficient torelax the glass lattice, but not cause densification of the glass, or tocause boiling of the ions in the ion bath.

In one embodiment, a glass article is chemically enhanced by immersionin a first and second ion bath. While immersed in the first and/orsecond ion baths the thermal profile of some predetermined portion ofthe glass article is increased through use of directed heating (coils,laser, microwave, etc.). The targeted locale on the glass articleundergoes additional ion exchange given the relaxed and expanded latticeof the glass. Once the thermal input is deemed sufficient, theasymmetrically strengthened locale, now having additional ions packedinto the surface, can be quenched to inhibit exchange of the ions backout of the locale. Increasing the thermal profile during chemicalstrengthening can be used to both increase the compressive stress of theglass surface and compressive stress layer depth of the glass surface.

Local Asymmetric Strengthening via Paste and Heat

As discussed in more detail below, it is often important to form a glassarticle where the stress in the glass article is matched to provide aparticular shape, for example, provide a flat surface.

In one embodiment, localized chemical strengthening techniques can beused to promote ion diffusion into specific regions or zones of theglass article. These high concentration chemical strengthening zones canbe used to instill higher surface ion concentration and/or deepercompression layers with target patterns or spots on the glass article.The inclusion of the enhanced chemical strengthening can be used toprovide slight curvatures to the glass surface where required, or can beused to counteract each other on opposite sides of the glass surface(front and back surface, for example).

Pastes that include high concentrations of potassium, for example, canbe used in combination with heat to enhance or promote ion diffusiondirectly from the paste into the localized surface of the glass article.This high concentration and direct ion diffusion is superior to the iondiffusion accomplished by immersion in ion baths. In one embodiment, aglass article, requiring an increased amount of ion diffusion in apredetermined pattern, is coated with a high ion concentration paste inthe predetermined pattern. The paste can be 30-100% molar sodium orpotassium nitrate for example, and more typically 75-100% molar. Thepaste layer thickness is determined by how much ion is required fordiffusion into the glass article surface. The coated glass article isthen placed in an oven and heated, for a predetermined amount of time,to increase the diffusion of the ion into the glass surface in thepredetermined pattern. Ovens can be electric or gas (or other like) andreach temperatures from about 250-500° C. In some embodiments, the ovencan be under pressure, allowing for use of higher temperatures duringthe heating step (and thereby avoiding evaporated or boiled paste).

FIG. 12A and FIG. 12B illustrate the use of high concentration ionpastes 1200 on the front (12A) and back (12B) surface (1202 and 1204,respectively) of a cover glass 1206. Paste application patterns can beused to facilitate allocation of asymmetric strengthening, andcounterbalance stress added on the front cover with stress added to theback cover. In FIGS. 12A and 12B, illustrative front and back surfacepatterns are presented.

In other embodiments, the already enhanced coated glass article iscoated with the high ion concentration paste, potassium for example, andthen placed into the potassium ion bath. The coated glass article andion bath are then placed in the oven for heating, such that the pastedirectly deposits potassium to the glass surface, while the potassiumion bath allows for ionic diffusion to the non-coated or exposedsurfaces of the glass article.

Altering the ion concentration in the paste, the pattern of pasteapplication on the glass surface, the heating parameters of the paste,the coating thickness of the paste, provide various design options forcreating an asymmetrically strengthened glass article.

As can be imagined, paste with high ion concentrations can also becombined with masking, ion-barrier coatings and glass density to furtheroptimize the necessary chemical strengthening for a target glassarticle. Also, as can be imagined, paste with multiple ions can be usedas well as coating a glass article surface with one or more, two ormore, three or more, etc. different pastes, each having a different ionor ions concentration.

Electric Field Assisted Asymmetric Chemical Strengthening

As shown above, embodiments herein include asymmetric glassstrengthening during the chemical strengthening process. In thisembodiment, ion transport in the ion bath is preferentially increasedtoward a target surface of the glass article, thereby increasing thediffusion of the ions at the target surface. Increased concentration ofthe ion at a surface allows for an increase in the amount of ionincorporated into the glass surface, up to the glass article's ionsaturation point, as compared to the remainder of the article's surfacenot in-line with increased ion concentration.

Aspects of this embodiment are maximized by utilizing an ionconcentration, in the ion bath, that provides for chemicalstrengthening, but below the glass articles' ion saturation point. Inthis aspect, the electric field would significantly increase the ionconcentration at surfaces in-line with the preferential transport ofions across the electric field.

In an illustrative embodiment, an electric field is established in anappropriate ion bath to preferentially diffuse the ion across the targetsurface of the immersed glass article. As shown in FIG. 13, a glassarticle 1304 in need of asymmetric chemical strengthening is positionedin the ion bath 1300 between a positive 1306 and negative electrode1308. Electron flow through the external circuit 1310 allows the bathions, potassium for example, to flow toward the negative electrode andthereby into the front surface 1302 of the positioned glass article(shown as arrow 1312). The increase in ion concentration at the frontsurface of the glass article provides for an asymmetric strengthening ofthe front surface, as the front surface 1302 will have an increase inion diffusion, as compared to the back surface 1314 of the glass.

Alternative embodiments for the electric filed gradient includeperforming the preferential ion diffusion in combination with coil,laser, microwave or other thermal heating (shown as arrow 1316). In thisembodiment, a glass article 1304 is exposed to localized microwaveradiation 1316, for example, where increased chemical strengthening isrequired. The microwave radiation facilitates stress relaxation at thetarget surface 1302. A glass article surface receiving preferential iondiffusion in the ion bath due to the established electric field, mayhave additional ion diffusion into the surface where the microwaveradiation facilitates stress relation (provides more space for ions toenter the glass surface). As can be imagined, a glass article 1304 sotreated could have several different asymmetrically strengthened zones,the zone that was heated 1318 and in-line with the ions in the electricfield, zones not heated but in-line with the ions in the electric field1320, zones heated but not in-line with the ions in the electric field(not shown), and zones that are neither heated or in-line with ions inthe electric field (1322).

Asymmetric Strengthening via an Introduced Pre-Bend

Asymmetric strengthening can be introduced into the surface of a glassarticle by pre-stressing the glass prior to, and during, the enhancingand strengthening process. In one embodiment, the glass article isformed (molded, drawn, etc.) to have a pre-desired curvature. The formedglass article is placed under the correct force to maintain the form andthen chemically strengthened using the embodiments as described above.For example, the formed glass article is placed in the ion exchangebaths in the pre-stressed or formed shape. Since the glass is bent whilethe glass is being chemically strengthened, it is strengthened in anenhanced manner. So, for a curved or bent glass article, the chemicalstrengthening is primarily going to the outer, curved, surface (ionsmore easily diffuse into the stretched glass lattice), while thecompressed inner surface undergoes limited chemical strengthening.Different portions of the outer surface of the glass article may beselectively chemically strengthened, or chemically strengtheneddifferently, and/or the glass article can be bent selectively ordifferently to offset the asymmetric chemical strengthening of thedifferent portions. After the pre-stressed glass article is releasedfrom its pre-bend, the outer surface will have a greater amount ofstrengthening as compared to the inner, thereby showing an asymmetricstrengthening profile.

FIG. 14A-14E illustrate chemically strengthening a glass articleaccording to one embodiment. In FIG. 14A, the glass article 1400 isshown having a thickness T. The thickness T can be generally asdescribed throughout this disclosure (0.3-5 mm). The glass article 1400has an outer surface 1402 and an inner surface 1404.

In FIG. 14B, an ion-exchange coating (as discussed above) 1406 is coatedonto the inner surface 1404 of the glass article 1400. In this way, theion-barrier limits ion diffusion into the inner surface of the glassarticle.

In FIG. 14C, the glass article has been bent such that the bent glassarticle 1400′ is curved inward towards the inner surface 1404. Thebending of the glass article yields the glass article with curvature C.The curvature in the glass article 1400′ can be of varying degrees, andcan be imposed by force (a fixture) or by including a heated environment(slumped over).

In FIG. 14D, the bent glass article from FIG. 14C undergoes chemicalstrengthening to yield a glass article 1400″ having a strengthenedregion 1406. The chemically strengthened region 1406 is providedadjacent the outer surface 1402 and not adjacent the inner surface 1404.The chemically strengthened region extends inward from the outer surfaceto a depth of layer (DoL), which is deeper into the glass than the DoLat the inner surface (which is minimal or non-existent). Since the outersurface is chemically strengthened substantially more than the innersurface, the chemically strengthened glass article 1400″ can be referredto as being asymmetrically chemically strengthened.

FIG. 14E illustrates the chemically strengthened glass article 1400″′after completion of the chemical strengthening process. The glassarticle 1400″′ is depicted as planar, or at least substantially planar,following completion of the process. The completed glass article 1400″′has an outer surface 1402 with increased compression and an innersurface 1404 that was both bent inward and coated by an ion-exchangecoating to limit or eliminate chemical strengthening. In this profiledesign, the chemically strengthened glass article 1400″′ tends to wrapinward from the outer surface—meaning the outer surface compresses andexpands. In such case, the warpage due to the chemical strengthening ofthe outer surface but not the inner surface causes the curvature C to becountered. Consequently, the chemically strengthened glass article1400″′ no longer has a curvature as it had prior to the beginning of thechemical strengthening.

Asymmetric Strengthening Different Clad Layers

FIG. 15 illustrates another Embodiment herein which includes formingasymmetrically strengthened glass articles 1500 through immersion ofglass article clad layers 1502 in the chemical strengthening bath 1504,where each glass article in the clad layer has a different starting ionconcentration and composition. A clad layer having a first and secondglass article is then strengthened using the chemical strengtheningprocesses described herein to provide two glass articles with asymmetricstrengthening.

In one aspect, since the starting compositions of the two glass articlesare different, the exposed surface and edges of each glass article willincorporate available ions differently. The end result of the chemicalprocessing step will be two glass articles with a protected surface(internal to the clad layering) and a chemically modified exposedsurface and edges. Modification of the exposed surfaces can be made bymasking or coating, or other embodiments herein, as describedpreviously. Any number of articles can be strengthened in this way, forexample, in FIG. 15, three glass articles are being strengthened at thesame time.

Chemical Strengthened Glass Article Bundles

In other aspects, asymmetrically strengthened glass articles havingsubstantially the same stress profiles can be bundled together forcommon treatment to alleviate or modify the stress in the bundled glass.Here, the glass articles can be bundled as multiple plates to oneanother and treated together to maximize efficiency. Glass articles canbe bundled as non-planar parts, treated, and then bonded to display abonding stress or could be pre-bent and then bonded to display thebonding stress.

Asymmetric Strengthening Glass Articles Having a Concentration Gradient

In another embodiment, two glass articles of differing composition canbe fused together prior to the chemical strengthening process. Here, thefused glass article will have a top surface chemically strengthenedbased on its starting glass ion concentration and composition (topglass), and a bottom surface chemically strengthened based on itsstarting glass ion concentration and composition (bottom glass).

In addition, using the same premise, one glass piece having aconcentration gradient (composition or ion) can also be chemicallystrengthened to provide asymmetrically strengthened glass. As above, theglass article has differing ions, at differing locations of the glassarticle, to be exchanged in the ion baths, such that the resultantsurface will be asymmetrically strengthened.

Design of the starting glass, including its starting ion concentrationsand locations, can therefore by used to calibrate the ion-diffused andasymmetrically strengthened glass.

Mechanical and/or Chemical Modifications to Tune a Stress Profile

Embodiments herein include the use of post-chemical strengthening,mechanical and/or chemical processes, to fine tune a glass article'sstress. Where a glass article has been prepared according to any of theembodiments described herein, fine tuning of the compressive stresslayer, for example, or tuning of the relationship between the tensileand compressive forces, in the glass may be required. Removal ofmaterial, either mechanically (grinding, polishing, cutting, etc.) orchemically (application of HF or other like acid), can be used tolocally modify the stress profile for the glass article.

For example, where it is determined that the extent of the compressivesurface stress layer is too large, or deep, removal of some amount ofthe layer will relieve stress and re-calibrate the stress profile forthe glass article. These post-chemical strengthening embodiments areparticularly useful where the stress modification need only be minor,for example removal of 10 μm from a limited region of the cover glass.

Asymmetric Chemical Strengthening During Glass Article Production

Embodiments herein include the stepwise modification of a glass articlesstress profile based on the use of one or more of the chemicalstrengthening embodiments described herein. For example, whereproduction of a glass article results in a non-conforming orunsatisfactory result, the asymmetric chemical strengthening embodimentsdescribed herein can be used to reform the stress so as to bring theglass article into compliance. This may entail localized asymmetricchemical strengthening, or conversely, removal of material, with theobject of adding or removing stress where necessary to correct anydefects in the glass article.

FIG. 16 illustrates one flow diagram including the process forasymmetrical chemical strengthening during glass article production1600. A glass article having already been assigned a particularcalibrated stress pattern 1602 is appropriately treated using any of theembodiments herein described 1604. The reliability and safety of theglass is tested by determining whether the glass cover exhibits thecorrect strengthening parameters 1606. Where the glass article conformsto the asymmetric chemical strengthening, the glass article is submittedfor its use 1608. Where a glass article fails to exhibit its appropriatechemical strengthening, it is passed through the processes andembodiments described herein to reapply the appropriate chemicalstrengthening and tested 1610. This process can be repeated as manytimes as necessary to obtain a glass article that conforms to thestandards of its use.

As such, embodiments herein include monitoring and correction of a glassarticles predetermined stress profile. Correction can include a numberof stress modifying iterations until the desired glass article stressprofile is obtained.

Asymmetric Chemical Strengthening to Manage a Fracture Pattern

Embodiments herein include asymmetrically strengthening a glass articleto exhibit or manage a predetermined fracture pattern. FIGS. 17A and 17Bshow illustrative chemical strengthening 1706/1708 applied to a coversheet 1704 to minimize fracture propagation (17A) or minimize cornerdamage 1710 (17B).

FIG. 18 shows a surface stress (CS) to distance graph illustrating thatpoints of tension 1800 can be developed along the surface of a glassarticle where a fracture would be more likely to occur than at points ofhigh surface stress 1802.

Using any of the embodiments described herein, an optimal fracturepattern for the particular glass article use can be developed.Embodiments include positioning the amount of surface compressionstress, the depth of compression stress, the top surface to bottomsurface tensile to compressive stress, and the planar tensile tocompressive stress, in an optimized pattern. A glass article can becalibrated to control the fracture pattern upon damage or excessive wearand tear by identifying and then incorporating the necessary compressivesurface stress, depth of stress and tensile stress, so as to facilitatea fracture in some regions, should one occur, as compared to otherregions. In this way, a crack could be encouraged along a perimeter ascompared to the center of the cover glass, for example. In one example,more significant tensile stress is positioned in a desired fracturelocation 1706 or 1710 as compared to locations of less preference. Crackdevelopment and propagation can be managed by the irregular use andpositioning of stress 1706, for example.

Designing a Cover Glass to Reduce Damage, or the Propagation of Damage,Caused by an Impact

Embodiments herein result in the production of asymmetricallystrengthened cover glass for a portable electronic device. As previouslydisclosed, the combination of stresses on the cover glass are budgetedto avoid failure and maintain safety, i.e., with a limited volume ofglass, only so much ionic material may be added to the volume before theglass will crack or fail, simply due to the tensile stress becomingoverly voluminous and exerting sufficient pressure to crack the glass.

In embodiments herein, asymmetrically strengthened cover glass has astress budget optimized to resist damage caused by impact from dropping,fumbling, hitting, and the like of the device, e.g., a mobile phonedropping from the users hand and falling to the floor. In this light,most portable devices, when impacted, tend to initially impact at acorner of the device, or to a lesser extent, impact at a perimeterstraight edge of the device. The impact is therefore aligned with thecorners of the cover glass and, to a lesser extent, the perimeter oredge of the cover glass. It is less likely, and more infrequent, that adropped device will initially impact at the front side or back side ofthe device, i.e., land flat on its face or flat on its back. As such,embodiments herein are optimized to limit or reduce damage (or thepropagation of damage) in a cover glass by designing the cover glasswith the expectation that impact will result at a corner of the coverglass, or at the very least, a perimeter straight edge of the coverglass.

As discussed previously, asymmetric chemical strengthening can be usedto provide modified surface compression within a cover glass. Theasymmetric strengthening must conform to a stress budget for theparticular parameters of the glass. Embodiments herein include coverglass designs where the stress budget is utilized to provide the mostimpact resistance at the cover glass corners, followed by impactresistance along the straight perimeter edges, and to a lesser extentthe substantially flat front and back surfaces of the glass. Thebudgeted stress is therefore substantially utilized at the corners, andto some extent, along the perimeter of the cover glass. Little or nostress budget is allocated to the center or remainder zone of the coverglass. The imparted strengthening is adequate to enhance impactresistance from damage. In addition, since little of the stress budgetis used in the center or remainder zone of the cover glass, that zone isunder little to no imbalance and can remain substantially flat.

FIG. 19 shows an illustrative flow diagram 1900 for asymmetricallystrengthening a glass article having multiple zones, each zone having adifferent stress profile. In operation 1902, a glass article is obtainedfor a desired utility based on its dimensions, its thickness, and itsinherent composition. In operation 1904, a budget for how much stressthe identified glass can withstand is determined based on the glass'sutility, and a budget determined for enhanced resistance to impactdamage caused by a drop, for example. As described throughout, thebudget must conform to the restricted volume of the glass, as inclusionof too much stress in the glass can cause the tensile stress to lead tocracks or damage under normal use constraints.

In operation 1906, the glass article is then divided into multiplezones. For example, a first zone in the glass may have the highestamount of chemical strengthening, followed by a second zone, followed bya third zone having the least amount of chemical strengthening. Inoperation 1908, the glass article has a stress pattern based on thethree different zones, for example, a first stress pattern having thegreatest strength related to impact, a second stress pattern having asmaller amount of strength than the first zone, and a third stresspattern having the lowest level of strength. In some embodiments thethird zone has little or no chemical strengthening.

FIG. 20 shows an illustrative flow diagram 2000 for asymmetricallystrengthening a cover glass for a portable electronic device havingthree or more zones, each zone having a different stress profile. Inoperation 2002, a cover glass is obtained having the dimensions,thickness and composition typically called for use in the portableelectronic device of interest. In operation 2004, a budget for how muchstress the cover glass can withstand is determined, where the budgetedstress maintains a substantially flat cover glass with enhanced damageresistance in the event of an impact, a drop for example. The coverglass can be divided into three zones, a first zone corresponding to thecorner portions or areas of the cover glass, a second zone correspondingto the straight perimeter portions (also referred to as peripheral edgeareas) of the cover glass, and a third zone corresponding to theremainder or center area of the cover glass. In some embodiments, thethree zones refer to the top surface of the cover glass, or to a stressprofile that extends from the top surface to the bottom surface. Thefirst and second zones can include up to 50% of the cover glass area(leaving 50% of the cover glass area for the third zone), up to 40% ofthe cover glass area (leaving 60% of the cover glass area for the thirdzone), up to 30% of the cover glass area (leaving 70% of the cover glassarea for the third zone), up to 20% of the cover glass area (leaving 80%of the cover glass area for the third zone), up to 15% of the coverglass area (leaving 85% of the cover glass area for the third zone), upto 10% of the cover glass area (leaving 90% of the cover glass area forthe third zone), up to 5% of the cover glass area (leaving 95% of thecover glass area for the third zone), up to 2.5% of the cover glass area(leaving 97.5% of the cover glass area for the third zone), and up to 1%of the cover glass area (leaving 99% of the cover glass area for thethird zone).

In typical embodiments herein, in operation 2006, the glass article canbe divided into a first zone that includes a first stress pattern usefulfor the corner portions of the cover glass, a second zone that includesa second stress pattern useful for the straight perimeter portions oredge portions of the cover glass, and a third zone that has a stresspattern useful for the remainder of the cover glass. In operation 2008,the budgeted stress is allocated to the three zones where the first zoneis strengthened more than the second zone, which is strengthened morethan the third zone. In some embodiments the third zone undergoes littleor no chemical strengthening, and the entirety of the stress budget isused on the first and second zones. Using the entirety of the stressbudget on the first and second zones results in a glass article that isunder tensile stress for normal usage, but has improved capacity toprevent or reduce damage caused by an impact to the article. Also notethat the first and second zones can form a continuous perimeter aroundthe third zone.

FIG. 21 illustrates a cover glass 2100 having three zones, each zonehaving a stress pattern useful in reducing damage, or the propagation ofdamage, in a cover glass. As noted above, a finite stress budget existsfor the cover glass 2100. The stress budget is allocated to each of thethree zones, where the first zone 2102 (corresponding to the cornerportions or areas of the cover glass) receives the highest amount ofchemical strengthening, a second zone 2104 (corresponding to thestraight perimeter sides or peripheral edge areas) receives the secondhighest amount of chemical strengthening, and a third zone 2106 thatcorresponds to the center or remainder area of the cover glass 2100receives the least amount of chemical strengthening. In someembodiments, the third zone 1906 may undergo little or no chemicalstrengthening. The third zone 2106 can include an external surface wherea portion thereof is typically substantially flat, rather than theentirety of the third zone. The third zone 2106 is also surrounded bythe higher strengthened first 2102 and second 2104 zones, which form acontiguous perimeter around the third zone. The contiguous first andsecond zones forming at the periphery of the cover glass higher strengthglass that forms a protective barrier against impact to the lowerstrengthened glass found in the third zone. In some embodiments, thefirst zone and second zone each form an edge and the edges can contacteach other to form an oblique angle. The stress budget is used to reducepotential impact events from causing damage, or the propagation ofdamage, to the first zone 2102, and to some lesser extent, the secondzone 2104, while leaving the third zone substantially flat or unaffectedby warpage. At the least, impact is likely to be distributed to thefirst and second zones of the cover glass 2100, which form a perimeteraround and surround the centrally located third zone 2106. In addition,the first zone can be thermally heated to a temperature that allows forincreased chemical strengthening as compared to the same zone in theabsence of thermal heating. The second zone may also be thermally heatedduring asymmetric strengthening to also enhance or increase the amountof stress induced in the zone. Thermal heating is described throughoutthe current specification, but can be performed by microwave or laserheating. In some embodiments, the temperature of the thermal heating isbelow the densification temperature of the glass and in otherembodiments the temperature of the thermal heating is above thedensification temperature of the glass.

FIG. 22 shows a cross-sectional view along line 21-21′ in FIG. 21. Thefirst zone 2102 shows an increased amount of ions 2200 to a particulardepth and concentration as compared to the third zone 2106. The changein ion concentration along the first zone surface, and to particulardepths, modifies the glass internal stress relationship. The increasedchemical strengthening to the first zone provides additional compressivestress along the zone or portion of the cover glass most likely to havean impact. In FIG. 22 the first zone defines a curved edge, which inthis embodiment, extends from the top surface to the bottom surface ofthe cover glass. Note that this is also the zone of the cover glass mostat risk from impact, as it has a limited area to distribute force orenergy caused by the impact. The increase in the volume of ions at thecorner can thereby resist the force or energy imparted by the impact andreduce or prevent damage to the cover glass. Alternatively, the thirdzone 2106 has a much greater area to distribute a force associated withan impact, as well as being much less likely to be involved with theimpact itself. As such, some of the chemical strengthening not requiredin the third zone can be budgeted to the first zone and still maintain acover glass within its budgeted amount of stress. As noted in FIG. 22,the third zone defines an external surface that is substantially flat.

Flattening Asymmetric Stress Profiles

Embodiments herein include the process of using asymmetric chemicalstrengthening, in combination with other compensating forces, to provideuseful glass articles, for example, articles having flat surfaces.

In one embodiment, a glass article that has been asymmetricallychemically strengthened shows a stress imbalance due to an overallexcess of compressive stress on the top surface as compared to thebottom surface, for example. The stress imbalance in the glass articlecan be counteracted by attachment to a very stiff material, or a stiffmaterial having a geometry that counteracts the stress imparted by theasymmetrically strengthened glass article. Optimal materials wouldcounteract the glass article's imparted asymmetric stress so as toremain flat (or remain at the geometry required for the glass material).In typical embodiments, the stiff material would be attached along thesurface of the glass article, typically the bottom surface. In somecases, the stiff material would be transparent. The stiff material wouldonly need to be of sufficient amount and coverage to accomplish thecounteracting stress.

In another embodiment, a glass article that has been asymmetricalchemical strengthened has its stress imbalance counterbalanced bytailoring mechanical or chemical removal of material. In thisembodiment, polishing or other mechanical technique can be used tooptimally remove stress from the glass article. Alternatively, aspectsof the glass article's stress imbalance can be removed by immersion ofthe part in chemical removal bath, e.g., an HF bath. Glass surface notat issue in the chemical removal bath could be sealed off from the HF oronly selective regions of the glass surface exposed to the HF. Removalof material would be accomplished to provide a glass article with thecorrect geometry or flatness (again based on counterbalancing theoverall stress in the strengthened glass article).

In still another embodiment, the required asymmetric compressive stress(for damage control and reliability) is counteracted by the introductionof additional, localized, chemical strengthening. For example, use ofcoatings or pastes (previously described) can be incorporated into theasymmetrically strengthened glass article to counteract the warpageintroduced by the required asymmetric chemical strengthening. In someaspects, the coatings or pastes can be patterned.

Embodiments herein also include not just the placement of counteractingchemical strengthening, but include the amount of compressive surfacestress and the depth of compression of the chemical strengthening on theglass. Here, the inclusion of a particular compressive surface stresscan act as a stiffening barrier to prevent or counteract warpageintroduced by other asymmetric chemical strengthening. Use of a short,high spike of potassium ions, for example, into the surface of the glassarticle can act to provide a very shallow but hard sport. These hard(high compressive surface stress layers) can have a Young's Modulus ashigh as 60 to 80 GPa and be used to prevent warpage—in a sense, act asthe stiff material discussed above.

Compensating Asymmetric Chemical Strengthening with Forming

Embodiments herein include the design and production of glass articlesthat combine the advantages of asymmetric strengthening of surfaces on aglass article, with glass forming.

As is described throughout the present disclosure, asymmetric chemicalstrengthening allows for targeted increases in either the compressivesurface stress of a glass article and/or the depth of compression of aglass surface. In most cases, the glass article is calibrated to haveits intended utility with maximum damage or scratch protection for theglass article. This typically requires some combination of the processesand embodiments described herein, for example, increased depth ofcompression along the perimeter of a cover glass with normal symmetricchemical strengthening in the center of the cover glass.

Inclusion of asymmetric chemical strengthening, however, can introducestress imbalance into the glass article (note the stress profilesdiscussed above). When enough stress imbalance is introduced to a glassarticle, the glass article will warp. Warpage in a glass article istypically detrimental to the article's utility and presents a limitationon how much asymmetric stress can be introduced into a glass article.

As previously discussed, introduced warpage can be compensated for byintroduction of competing stress imbalances, for example, introducingasymmetric chemical strengthening in a glass article so as to bothprovide utility and to provide counteracting stress. The presentembodiment, however, utilizes the glass forming process to minimize thestress imbalances introduced by asymmetric chemical strengthening.Further, glass forming provides a stiffer glass article which can beformed to combine with the forces incurred through asymmetric chemicalstrengthening to yield a glass article having the desired shape.

In one embodiment, a glass article is designed to counteract the stressimbalances introduced by asymmetric chemical strengthening with the useof glass forming. In one aspect the asymmetric chemical strengthening iscounteracted by forming the glass article with an appropriate geometry.Appropriate glass article geometries for a particular stress profileprovide stiffness to counteract the stress introduced by the asymmetricchemical strengthening procedures. In an alternative embodiment, theasymmetric chemical strengthening is combined with glass forming toprovide a desired geometry, for example, the warpage of thestrengthening is combined with glass forming curvature to yield adesired shape.

Where a desired glass article shape entails a non-uniformcross-sectional shape, or thickness, symmetric chemical strengtheningwould actually contribute to a wider-spread potential warpage.Asymmetric chemical strengthening allows for both inclusion of thedesired compressive stress layers and depth and avoids the significantwarpage. Glass forming combines with the strengthening to provide anoptimized glass article.

FIG. 23 is a flow chart illustrating that a glass article can beidentified and formed with the appropriate local stiffness to counteractthe proposed asymmetric chemical strengthening 2300. The formed glass2302 can undergo CNS and polishing 2304. The glass article thenundergoes the various steps required to introduce the asymmetricchemical strengthening, including, for example, the use of barrierlayers, pastes, heat, etc. (2306, 2308, 2310, 2312, 2314 and 2316). Theformed glass article with enhanced stiffness can be treated multipletimes to obtain a highly calibrated surface or surface.

Optimized Glass Article Design Based on Stress Distribution

Embodiments herein include processes for calibrating the strength of aglass article for a particular use using any one or more of: pre-heatinga glass article to a higher glass density, modifying the edge geometriesof a glass article to maximize geometric strengthening, modifiedchemical strengthening using masking, ion barrier or limiting coatings,chemical strengthening using ion enhancing pastes and heat, thermallyassisting the chemical strengthening, directed or preferred iondiffusion using electric fields and heat, introducing pre-stress totarget articles, and tuning the stress found in asymmetrically preparedglass articles.

Calibration can also occur during the glass manufacturing process, forexample, through differential strengthening of glass in clad layers,through identification of useful ion gradients and concentrations instarting glass, and through fusing glass articles together, and thelike.

Aspects herein utilize each of the above embodiments to calibrate aglass article, having a budgeted amount of stress, in the vertical andhorizontal axis. Budgeted and irregular stress allows for placement ofcompressive stress layers of predetermined hardness and depth on thefront, back, top, sides and edges of a glass article to both optimizethe reliability of the glass article and to make the glass article safefor its intended use. Budgeted irregular stress in the glass article canalso be offset by counteracting stress input by other materials, or bythe geometry of the glass itself. This is particularly useful when thefinished glass article is designed to be flat or other targetedgeometry. In this manner a glass cover, for example, can be evaluatedfor its intended use, i.e., how much surface compressive stress does thearticle require on the top surface, bottom surface, edges, etc., howdeep does the compressive stress need to extend at each of these zones,how much tensile strength will result from these compressive stressneeds, how much tensile strength will result, can the required stressesbe balanced using chemical strengthening alone, can glass forming beused, and the like. Embodiments herein are then utilized to perform thecalibration to provide a high utility glass cover with maximized oroptimized value.

The following Example is provided for illustrative purposes only and isnot intended to limit the scope of the disclosure.

Example of Glass Forming to Compensate for Asymmetric ChemicalStrengthening

The depth of compression in ion-exchange chemical strengthening iscorrelated to the ability of a glass article to resist failure by damageinduction. In this light, maximizing the depth of compression is asignificant driver in producing more durable and reliable glass for usein portable electronic devices.

Depth of compression in a glass article saturates once the ions havebeen diffused through the thickness of the glass. This shows thatasymmetric strengthening can be used to achieve a deeper depth ofcompression, and thereby facilitate a glass article's ability to resistfailure. Further, although asymmetric strengthening introduces warpagevia a stress imbalance in the glass article, the warpage can becompensated for by using glass forming.

The use of glass forming includes using stiffer cover glass designs, aswell as forming a cover glass geometry to compensate for the introducedasymmetric warpage. For example, glass forming can be used to compensateor exacerbate the asymmetric chemical strengthening stresses to ensurethat the combined procedures results in the desired final part shape.

Depth of compression can be implemented into a cover glass by using oneor more of the asymmetric chemical strengthening processes describedherein. FIG. 24 provides an example of an asymmetrically strengthenedcover glass chemically strengthened to a greater depth of layer at theexterior surface and to a greater compressive surface stress at theinterior surface. As shown in FIG. 24, cover glass 2400 includes a firstcompressive stress region 2408 which extends from exterior surface 2406to a first depth DoL₁ and has a first compressive surface stress CS₁. InFIG. 24, the first depth DoL₁ extends about to a midpoint of thethickness of the cover glass 2400. The glass cover further includes asecond compressive stress region 2412 which extends from interiorsurface 2410 to a second depth DoL₂ and has a second compressive surfacestress CS₂. As shown, the first depth DoL₁ is greater than the seconddepth DoL₂ and the first compressive surface stress CS₁ is less than thesecond compressive surface stress CS₂. In embodiments, the surfacecompressive stress CS₂ along an interior surface of the cover glass 2400may be from 600 MPa to 800 MPa and the surface compressive stress CS₁along an exterior surface may be from 300 MPa to less than 600 MPa. Inadditional embodiments, the first depth DoL₁ may be from 75 microns to175 microns and the second depth DoL₂ may be from 10 microns to 50microns. In further embodiments, the first depth DoL₁ may be from 1.5 to8 times or from 1.5 to 5 times the second depth DoL₂. The cover glass2400 has a corresponding, but budgeted amount of tensile stress 2418 tooffset the exterior and interior asymmetric surface compression.

FIGS. 25-30 illustrate one such asymmetric chemical strengthening andglass forming procedure.

In FIG. 25, a glass cover is obtained and undergoes CNC to fit its basicdesign needs. A cross section view shows the initial cover glassgeometry. FIG. 25 shows that glass forming can be used to introduce abend 2502 (via bending stress) at the end of the cover glass 2500. Notethat symmetric chemical strengthening of this formed glass would resultin a highly warped glass article, and provide little value.

In FIG. 26, the cover glass 2600 can undergo further CNC and polishingto further prepare the cover glass. Next, in FIG. 27, the bottom flatsurface 2702 of the cover glass 2700, up to the formed bend, is coatedwith a layer of an ion-exchange diffusion barrier, silicon nitride (e.g.SiN) 2704. The silicon nitride will significantly limit ion diffusionthrough the flat bottom surface of the cover glass. This will furtherensure that the covered surface remains substantially flat.

The formed and partially masked cover glass is treated under thechemical strengthening process described herein in FIGS. 28A and 28B. Ascan be seen from FIG. 28A, a cross section of the glass 2800 indicatesthat the top surface 2802 of the cover glass has a compression layer ofdepth DoL formed by diffusion of potassium 2803. The bottom surface2804, coated by the silicon nitride has no, or very minimal, chemicalstrengthening, as expected. FIG. 28B shows a cross-sectional view of thestatus of the formed cover glass 2800.

FIG. 28C is the corresponding stress profile, where the top surface 2802of the glass cover 2800 shows a high compressive stress and significantDoL and the bottom cover 2804, where there was no strengthening, showsno compression and only tensile stress (that results from balancing thestress at the top surface).

FIGS. 29A and 29B illustrate that the silicon nitride layer on thebottom surface of the glass cover 2900 can be oxidized to SiO₂ 2902,which is no longer a complete barrier to chemical strengthening. Asecond round of chemical strengthening is performed on the formed glasscover to provide the cross sectional view shown in FIG. 30A. Note thatthe bottom surface 3004 now includes a shallow compressive layer, whilethe top surface 3002 has been further enhanced with a higher surfacecompression (FIG. 30A).

Finally, FIG. 30B illustrates the final cover glass 3000 that includes acover glass geometry to complement an asymmetric stress profile from theseries of chemical strengthening procedures. The cover glass hasexcellent top cover surface compression 3002 and DoL, matched by thegeometry and high compressive stress with limited DoL of the bottomsurface 3004 (see FIG. 30B).

FIG. 30C is the corresponding stress profile, where the top surface 3006of the glass cover 3000 shows high surface compression 3008. The bottomsurface 3010 shows some amount of surface compression 3012,corresponding to the lower allowance of chemical strengthening. Thecover glass 3000 has a corresponding, but budgeted amount of tensilestress 3014 to offset the top and bottom asymmetric surface compression.

Asymmetric Strengthening of Glass Articles with Non-Uniform Thickness

As discussed briefly above, an asymmetrically chemically strengthenedglass article may have portions of different thickness from one another.Generally, thicker portions of the glass article may be chemicallystrengthened to a deeper depth of layer (DoL) than thinner portions. Byasymmetrically chemically strengthening the glass, the thicker portionsmay be made more crack- and/or impact-resistant than the thinnerportions, while ensuring that no portion of the glass article shattersor releases fragments upon impact or cracking. Thus, the glass articlemay have portions with different degrees of chemical strengthening, suchthat thicker portions may be more crack resistant while thinner portionshave some, but less, crack resistance.

In some embodiments, a peripheral portion of the glass article may bethicker while a central portion is thinner, for example to provideadditional space in an electronic device enclosure to accommodateinternal components of the electronic device. The peripheral portion,which is more likely to be impacted as a result of dropping theelectronic device, may have its resistance to cracking, chipping, orotherwise breaking enhanced by a deeper chemical strengthening layer (asmeasured from an exterior surface of the glass article) whilemaintaining overall safety and reliability of the glass article.

FIGS. 31-34B illustrate sample embodiments of glass articles havingportions of different thickness and each will be discussed in turn. Inembodiments, a first portion of the glass article may have a firstthickness and the second portion of the glass article may have a secondthickness greater than the first thickness. In further embodiments, acentral zone of the glass article may have the first thickness and aperipheral zone of the glass article may have the greater secondthickness. In additional embodiments, the central zone of the glassarticle may define the first portion and the second portion and theglass article may further comprise a peripheral zone.

The thickness of a given portion of the glass article may be definedbetween a region of an exterior surface and a corresponding region ofthe interior surface. For example, a thickness may be defined between afront region of the exterior surface and a corresponding rear region ofthe interior surface. When a region of a surface defines a curve, athickness may be defined along a perpendicular to the curved region ofthe surface.

In further embodiments, the glass article may define a lateralthickness. For example, when the glass article includes a thinnercentral portion and a thicker peripheral portion, the peripheral portionmay define the lateral thickness. In particular, when the interiorsurface varies in elevation, as illustrated in FIGS. 32A-33B and 34B,the peripheral portion of the glass article may define a lateralthickness between a side region of the exterior surface (i.e. a sidesurface) and a corresponding transition region of the interior surface.

FIG. 31 is a top view of a sample glass article 3100, showing a thickerperipheral portion 3125 and a thinner central portion 3120. Centralportion 3120 and a region of peripheral portion 3125 are generallycoplanar at exterior surface 3102. The transition between the peripheralportion 3125 and central portion 3120 is shown by a phantom linecorresponding to the step transition in thickness at the interiorsurface illustrated in the cross-section of FIG. 32A, discussed below.The thicker peripheral portion 3125 and thinner central portion 3120 arecontiguous with one another, meaning that the two regions or portionsabout one another with no intervening structure.

The central portion 3120 shown in FIG. 31 is bounded by the peripheralportion of the glass article 3125 and has width W₁ and length L₁ Centralportion 3120 is not confined to a midpoint of the glass article, butextends to an interior surface and an exterior surface of the glassarticle, as shown in FIG. 32A. An exterior surface of central portion3120 may be generally planar.

In embodiments, a peripheral portion of the glass article includes aperiphery of the glass article. By the way of example, the peripheralportion may include a side surface of the glass article. As illustratedin FIG. 31, peripheral portion 3125 includes corner portion 3127 andremainder portion 3129. Peripheral portion 3125 has a lateral thicknessX₃ in corner portion 3127 and a lateral thickness X₂ in remainderportion 3129. The lateral thicknesses X₂ and X₃ may be measured along aplane parallel to that of the central portion. As shown, the lateralthickness X₃ in the corner zone may be greater than the lateralthickness X₂ in the remainder portion. In addition, lateral thickness ofthe peripheral portion of the glass may be greater than a thickness ofthe central portion. In embodiments, an exterior surface of peripheralportion 3125 is generally coplanar with an exterior surface of centralportion 3120. In some embodiments, the exterior surface of peripheralportion 3125 includes a curved region.

The glass article 3100 may be used as a cover glass, portion of ahousing, input surface, and so on of an electronic device. Accordingly,the shape and/or dimensions of the glass article 3100 are intended toillustrate general concepts as opposed to a particular requirement. Theglass article 3100 may be transparent, translucent or opaque.

FIG. 32A is a cross-section taken along line 31-31 of FIG. 31 of anexample glass article 3200. The thicker peripheral portion 3225 andthinner central portion 3220 of the glass article 3200 are contiguous.As shown, the peripheral portion 3225 steps down to the central portion3220 (e.g., transitions to a lesser thickness).

The exterior surface 3202 of the glass article includes a centralexterior surface 3202 a in the central portion and peripheral exteriorsurface 3202 b in the peripheral portion. The interior surface 3204 ofthe article includes central interior surface 3204 a in the centralportion, peripheral interior surface 3204 c in the peripheral portion,and transitional interior surface 3204 b at the transition in thicknessbetween the central portion and the peripheral portion. Alternately,transitional interior surface 3204 b may be referred to as a wallsurface. As shown, central exterior surface 3202 a is generally oppositecentral interior surface 3204 a and peripheral exterior surface 3202 bis generally opposite peripheral interior surface 3204 c. One edge 3210provides a transition between the peripheral exterior surface 3202 b andside surface 3208 while another edge 3210 provides a transition betweenperipheral interior surface 3204 c and side surface 3208.

As shown in FIG. 32A, the central portion 3220 of the glass articledefines central exterior surface 3202 a and central interior surface3204 a. The central portion 3220 has a thickness T₁ between centralexterior surface 3202 a and central interior surface 3204 b. Thethickness T₁ shown in FIG. 32 is substantially constant, but in furtherembodiments the thickness in central portion 3220 may vary.

The peripheral portion 3225 of the glass article of FIG. 32A definesperipheral exterior surface 3202 b, edges 3210, side surface 3208,peripheral interior surface 3204 c, and transitional interior surface3404 b. The peripheral portion has a thickness T₂ between peripheralexterior surface 3202 b and peripheral interior surface 3204 c. Inaddition, the peripheral portion has a lateral thickness X₂ betweentransitional interior surface 3204 b and side surface 3208. Inembodiments, thickness T₁ is at least 10%, 20%, 30%, 40%, or 50% ofthickness T₂ The thickness T₁ may be up to 70% of thickness T₂. Thelateral thickness X₂ may also be greater than T₁. In embodiments,thickness T₁ is 20%, 30%, 40% , 50%, 60% or up to 70% of lateralthickness X₂ When the lateral thickness X₃ of the corner portion isgreater than the lateral thickness X₂ of a remainder of the peripheralportion, the thickness T₁ may be 10%, 20%, 30%, 40%, or up to 50%, oflateral thickness X₃.

As previously discussed, the glass article may be chemicallystrengthened by ion exchange to form a compressive stress layer atsurfaces of the glass article. The depth of layer (DoL) and/or thecompressive surface stress (CS) may be different for different surfacesof the glass article. In additional embodiments, the compressive stresslayer may have the same DoL and CS at some surfaces of the glassarticle. Further, as previously described, formation of a compressivestress layer along surfaces of the glass article creates a tensilestress region inside the glass article which balances the compressivestresses.

In aspects disclosed herein, the glass article 3200 is asymmetricallychemically strengthened such that the depth of the compressive stresslayer (DoL) varies around the glass article 3200. In embodiments, adepth of the compressive stress layer in a thicker portion of the glassarticle is greater than a depth of the compressive stress layer in athinner portion of the glass article. In further embodiments, thecompressive surface stress (CS) may be different for different surfacesof the glass article 3200. FIGS. 32B-32C, 33B, and 34B illustrateexamples of compressive stress layers formed in glass articles havingportions of different thickness. In FIGS. 32B-32C, 33B, and 34B thecompressive stress layer is indicated by a dashed line and stippling,both of which are not intended to illustrate any particular material,ion density, or other quality other than depth of layer.

In embodiments, the compressive stress layer may differ in the centralportion and the peripheral portion of the glass article. In furtherembodiments, the compressive stress layer along the exterior surface ofthe glass article may differ in the central portion and the peripheralportion of the glass article. For example, a depth of the compressivestress layer along the exterior surface in the peripheral portion of thearticle may be greater than a depth of the compressive stress layeralong the exterior surface in a central portion of the article, asillustrated in FIG. 32C. In additional embodiments, a depth of thecompressive stress layer along the exterior surface of the peripheralportion of the article may be about the same as a depth of thecompressive stress layer along the exterior surface of the centralportion of the article, as illustrated in FIG. 32B.

In further embodiments, the compressive stress layer along a sidesurface of the peripheral portion may have a depth which is the same orwhich is different than the depth of the compressive stress layer alongat least one of an exterior surface and an interior surface of theperipheral portion. As one example, the compressive stress layer alongthe side surface 3208 may have a depth which is about the same as thedepth of the compressive stress layer along the exterior surface 3202 a,as illustrated in FIG. 32C. As another example, the compressive stresslayer along side surface 3208 may have a depth which is different that adepth of the compressive layer along each of exterior surface 3202 a andinterior surface 3204 c.

The compressive stress layer along an interior surface of the glassarticle at the transition in thickness between the peripheral portionand the central portion may be largely influenced by the compressivestress layer along the adjacent interior surface of the peripheralportion and the adjacent interior surface of the central portion, asillustrated in FIG. 32B. In embodiments, the compressive stress layeralong the interior surface at the transition in thickness may besignificantly different than the compressive stress layer along the sidesurface of the glass article. For example, a depth of the compressivestress layer along the interior surface at the transition in thicknessmay be less than a depth of the compressive stress layer along the sidesurface as shown in FIG. 32C.

In embodiments, a compressive stress layer along a region of theexterior surface in the central portion is different than thecompressive stress layer along a region of the interior surface in thecentral portion. For example, the compressive stress layer along centralexterior surface 3202 a may have a depth greater than a depth of centralinterior surface 3204 a. FIGS. 32B and 32C illustrate examples of suchcompressive stress layers 3230 formed in the central portion of theglass article. Tensile stress region 3240 is also shown.

In additional embodiments, the compressive stress layer along a regionof the exterior surface of the peripheral portion of the glass articleis different than the compressive stress layer along a region of theinterior surface. As one example, the compressive stress layer along theperipheral exterior surface 3202 b may have a depth (i.e., depth oflayer) less than a depth of the peripheral interior surface 3204 c, asillustrated in FIG. 32B. Alternately, the compressive stress layer alongthe peripheral exterior surface 3202 b may have a depth greater than adepth of peripheral interior surface 3204 c, as illustrated in FIG. 32B.

As previously discussed, the internal tensile stress region may have athickness which limits the central tension within the glass article. Inembodiments, the thickness of the internal tensile stress region may bereferenced to a thickness of the glass article. For example, thethickness of the internal tensile stress region in the peripheralportion of the glass article may be referenced to a thickness of theperipheral portion. In embodiments, the thickness of the internaltensile stress region in the peripheral region is at least 10%, 20%, or30% of the thickness between a peripheral external surface and aperipheral internal surface, such as thickness T₂ in FIG. 32B. Further,the thickness of the internal stress region in the central portion ofthe glass article may be referenced to the thickness of the centralportion. In embodiments, the thickness of the internal tensile stressregion in the central region is at least 10%, 20%, or 30% of thethickness between a central external surface and a central internalsurface, such as thickness T₁ in FIG. 32B.

FIG. 32B illustrates a cross-section view of an example glass articlehaving a compressive stress layer 3230 extending around the glassarticle and an internal region of central tension 3240. For the glassarticle 3200 shown in FIG. 32B, the compressive stress layer 3230 has afirst depth (DoL₁) extending from central exterior surface 3202 a andfrom peripheral exterior surface 3202 b. The compressive stress layer3230 has a second depth (DoL₂) extending from the side surface 3208, athird depth (DoL₃) extending from peripheral interior surface 3204 c,and a fourth depth (DoL₄) extending from central interior surface 3204a.

As shown in FIG. 32B, each of the depths DoL₁-DoL₄ are different fromone another, indicating that the glass article has been asymmetricallychemically strengthened. Put another way, the DoL varies in differentportions of the glass article and/or as measured from different surfacesof the glass article. Accordingly, the thicker peripheral portion 3225has a deeper DoL along the interior surface of the glass article thandoes the thinner central portion 3220 Likewise, a region of thecompressive stress layer 3230 extending from the central exteriorsurface 3202 a is thicker than a region of the compressive stress layer3230 extending from central interior surface 3204 a (at the backside ofthe article 3220), but thinner than regions of the layer 3230 defined inthe peripheral portion 3225.

In some aspects of the invention, the compressive stress layer may bedescribed as comprising multiple regions. Each region of the compressivestress layer (i.e., compressive stress region) may be associated withone or more surfaces of the glass article. For example, the glassarticle of FIG. 32B may be described as having a first compressivestress region 3331 along central exterior surface 3202 a and peripheralexterior surface 3202 b, a second compressive stress region 3332 alongside surface 3208, a third compressive stress region 3333 alongperipheral interior surface 3204 c, and a fourth compressive stressregion 3334 along central interior surface 3204 a. Each compressivestress region has a depth of layer (DoL) and a compressive surfacestress (CS).

FIG. 32C illustrates a cross-section view of another example glassarticle having a compressive stress layer 3230 extending around theglass article and an internal region of central tension 3240. For theglass article 3200 shown in FIG. 32C, the compressive stress layer 3230has a first depth (DoL₁) extending from central exterior surface 3202 aand a second depth (DoL₂) extending from peripheral exterior surface3202 b and side surface 3208. The second depth is thicker than the firstdepth. Accordingly, the thicker peripheral portion 3225 has a deeper DoLalong the exterior surface of the glass article than does the thinnercentral portion 3220. As shown in FIG. 32C, the compressive stress layer3230 includes first compressive stress region 3231 extending fromcentral exterior surface 3202 a and second compressive stress region3232 extending from peripheral exterior surface 3202 b and side surface3208.

The compressive stress layer 3230 has a third depth (DoL₃) extendingfrom central interior surface 3204 a, transitional interior surface 3204b, and peripheral interior surface 3204 c. As shown, the third depth isthinner than each of the first and the second depth. For example, thethird depth may be from 25% to 75% of the first depth. In embodiments,the compressive stress layer along the interior surface of the glassarticle has a relatively high compressive surface stress even though thedepth of the layer is relatively small. By the way of example, thecompressive surface stress may be at least 75% of the compressivesurface stress at peripheral exterior surface 3202 b. As anotherexample, the compressive surface stress of the compressive stress layeralong the interior surface of the glass article may be greater than orequal to the compressive surface stress at peripheral exterior surface3202 b. In some embodiments, the surface compressive stress along aninterior surface of the glass article may be from 600 MPa to 800 MPa andthe surface compressive stress along a peripheral exterior surface maybe from 300 MPa to less than 600 MPa. The region of the compressivestress layer extending from central interior surface 3204 a,transitional interior surface 3204 b, and peripheral interior surface3204 c is labelled as third compressive stress region 3233 in FIG. 32C.

FIG. 33A is a cross-sectional view of another example glass article. Asshown, the glass article 3300 includes a thicker peripheral portion 3325and thinner central portion 3320. The exterior surface of peripheralportion 3325 comprises a first peripheral exterior surface 3302 b, whichis generally flat, and as second peripheral exterior surface 3302 c,which is curved. As shown, the thickness of peripheral portion 3325varies, but at least some sections of peripheral portion have athickness greater than the thickness T₁ of the central portion 3320.

One measure of the thickness in the peripheral portion is the distancefrom peripheral interior surface 3304 c to first peripheral exteriorsurface 3302 b along a perpendicular to peripheral interior surface 3304c, labeled as T₂ in FIG. 33A. A measure of the lateral thickness in theperipheral portion is a distance between transitional interior surface3304 b to side surface 3308 along a perpendicular to transitionalinterior surface 3304 b, labeled as X₂ in FIG. 33A. Each of X₂ and T₂may be greater than thickness T₁, as shown in FIG. 33A. When the glassarticle has a generally planar exterior region as shown in FIG. 33A, anaxis aligned with the generally planar exterior region may be referredto as a horizontal axis. In this case, X₂ may be referred to as athickness in a horizontal direction and T₂ may be referred to as athickness in a vertical direction.

The curve defined by the second peripheral exterior surface 3302 c mayspan a substantial fraction of a thickness of the peripheral region. Asexamples, the horizontal distance spanned by the curve may be at least30%, 40%, or 50% of distance X₂ In addition, the vertical distancespanned by the curve may be at least 20%, 30%, or 40% of distance T₂. Asshown in FIG. 33A, the second peripheral exterior surface 3302 c mayadjoin side surface 3308. Alternately, the second peripheral exteriorsurface 3302 may adjoin peripheral interior surface 3304 c.

As shown, the peripheral portion 3325 steps down (e.g., transitions inthickness) to the central portion 3220, with the transition in thicknessbeing along the interior surface of the glass article. The interiorsurface of the article includes central interior surface 3304 a in thecentral portion, peripheral interior surface 3304 c in the peripheralportion, and transitional interior surface 3304 b at a transition inthickness between the central portion and the peripheral portion.Central exterior surface 3302 a is generally opposite central interiorsurface 3304 a.

FIG. 33B shows another cross-section view of the glass article of FIG.33A. As shown in FIG. 33B, the glass article has a compressive stresslayer 3330 extending around the glass article and an internal region ofcentral tension 3340. The compressive stress layer 3330 extends fromcentral exterior surface 3302 a to a first depth (DoL₁). The depth ofthe compressive stress layer 3330 extending from first peripheralexterior surface 3302 b transitions from the first depth to a seconddepth (DoL₂) which is greater than the first depth. In embodiments, thecompressive stress layer 3330 extends from curved second peripheralexterior surface 3302 c to the second depth. The deeper compressivestress layer serves to protect the curved peripheral exterior surfacefrom damage.

The compressive stress layer 3330 has a third depth (DoL₃) extendingfrom central interior surface 3304 a, transitional interior surface 3304b, and peripheral interior surface 3304 c. As shown, the third depth isthinner than each of the first and the second depth. For example, thethird depth may be from 25% to 75% of the first depth. In embodiments,the compressive stress layer along the interior surface of the glassarticle has a relatively high compressive surface stress even though thedepth of the layer is relatively small. By the way of example, thecompressive surface stress may be at least 75% of the compressivesurface stress at peripheral exterior surface 3302 b. As anotherexample, the compressive surface stress of the compressive stress layeralong the interior surface of the glass article may be greater than orequal to the compressive surface stress at peripheral exterior surface3302 b. In some embodiments, the surface compressive stress along aninterior surface of the glass article may be from 600 MPa to 800 MPa andthe surface compressive stress along a peripheral exterior surface maybe from 300 MPa to less than 600 MPa. FIG. 33B also labels differentregions of the compressive stress layer including first compressivestress region 3331 extending from central peripheral surface 3302 a,second compressive stress region 3332 extending from second peripheralexterior surface 3302 c, and third compressive stress region 3333extending from peripheral interior surface 3304 c, transitional interiorsurface 3304 b, and central interior surface 3304 a.

FIGS. 34A-34B illustrate another example embodiment of a glass article3400. This glass article 3400 is suitable for any use as describedherein and specifically including those discussed with respect to FIG.31. As with the glass article 3100 shown in FIG. 31, the glass article3400 includes a peripheral portion 3425 and a central portion 3420. Thecentral portion includes thinner portions (first portions 3420 a) andthicker portions (second portions 3420 b). The thicker portions defineat least one rib feature. The peripheral portion 3425 is also thickerthan portions 3420 a, the thicknesses are illustrated in thecross-section view of FIG. 34B. Therefore, the thicker portions of theglass article include the peripheral portion 3425 as well as portions3420 b of the central region.

As illustrated in the top view of FIG. 34A, portions 3420 b of thecentral region may be viewed as forming a series of rib features. Therib features 3420 b of FIG. 34A are contiguous with peripheral portion3425 a. Rib features 3420 b extend to the peripheral portion 3425 a and,as shown, span the central zone of the glass article. The multiplethinner portions 3420 a are defined by thicker peripheral portion 3425and thicker portions 3420 b. Each of the thinner portions 3420 a may beviewed as forming island features, each of which are separated from oneanother by a thicker portion 3420 b.

FIG. 34B illustrates a cross-sectional view of the glass article 3400,taken along line 34-34 of FIG. 34A. As shown in FIG. 34B, the centralportion includes thinner portions 3420 a and thicker portion 3420 b. Theperipheral portion 3425 is also thicker than portions 3420 a. In someembodiments, a thickness of peripheral portion 3425 is about the same asa thickness of portions 3420 a.

Also similar to glass article 3100, the DoL of the asymmetricallychemically strengthened compressive stress layer 3430 is deeper atthicker portions (e.g., portions 3420 b and 3425) and is shallower atthinner portions (such as portions 3420 a). Likewise the DoL may bedifferent at front and rear surfaces of the same portion of the glassarticle 3400 or as the layer extends inward from a side surface 3408. Itshould be appreciated, then, that multiple thinned and/or thickenedportions may each exist in a single glass article and the DoL of anasymmetrically chemically strengthened layer may be different as betweenany or all such portions or regions.

As shown in FIG. 34B, the central zone comprises a first portion 3420 ahaving a first thickness T₁ and a second portion 3425 b having a secondthickness T₂ greater than the first thickness T₁. As shown, the firstportion 3420 a comprises a first front surface 3402 a and a first rearsurface 3404 a and the second portion comprises a second front surface3402 b and a second rear surface 3404 b. Further, the second portioncomprises wall surface 3404 d which adjoins the first rear surface 3404a and the second rear surface 3404 b. Alternately, the wall surface 3404d may be referred to as a transitional surface as it provides atransition between first rear surface 3404 a and the second rear surface3404 b.

The first portion further comprises a first compressive stress region3431 having a first depth DoL₁ along the first front surface; and asecond compressive stress region 3432 having a second depth DoL₂, lessthan the first depth, along the first rear surface. The second portion3420 b comprises a third compressive stress region 3433 having a thirddepth DoL₃ along the second front surface. The second portion furthercomprises a fourth compressive stress region 3434 having a fourth depthDoL₄ along the second rear surface 3404 b.

The peripheral zone 3425 has a third thickness T₃, which is shown assubstantially equal to second thickness T₂. The peripheral zone 3425comprises a third front surface 3402 c and a third rear surface 3404 c.The peripheral zone 3425 further comprises wall surface 3404 e. Asshown, the wall surface 3404 adjoins and provides a transition betweenthird rear surface 3404 c and first rear surface 3402 a. In furtherembodiments, the thickness T₃ may be greater than the second thicknessT₂.

As shown, the peripheral zone 3425 further comprises a fifth compressivestress region 3435 having a fifth depth DoL₅ along the third frontsurface 3402 c; and a sixth compressive stress region 3436 having asixth depth DoL₆ along the third rear surface 3406 c. As shown in FIG.34C, each of the third depth DoL₃ and the fifth depth DoL₅ aresubstantially equal to the first depth DoL₁. However, this is notlimiting and the third depth and/or the fifth depth may be differentfrom the first depth. For example, the fifth depth may be greater thanthe first depth.

As shown in FIG. 34B, the peripheral zone 3425 further comprises a sidesurface 3408 and a seventh compressive stress region 3437 having aseventh depth DoL₇ along the side surface. As shown, the seventh depthis substantially equal to the first depth, but this is not limiting. Insome embodiments, the seventh depth may be greater than the first depth.

Likewise, locally thinned or thickened portion of the glass article candefine islands, protrusions, bosses, steps, plateaus, undercuts, or anyother structural feature. A glass article may be asymmetricallychemically strengthened to different depths of layer in any or all suchstructural features, as described herein. Likewise, thicker regions orportions of a glass article need not surround thinner regions orportion, nor are thicker regions/portions necessarily at one or moreedges of a glass article. Any relative positioning, size, shape, and/ordimension of thicker and thinner portions or regions is contemplated.

It should be appreciated that asymmetric chemical strengthening,including asymmetric chemical strengthening in which the DoL is deeperin at least some thicker portions of a glass article than it is inthinner portions, may be created by selectively masking certainsections, performing multiple chemical strengthening operations incertain sections, discretely treating certain sections, or otherwisethrough any operation or method disclosed herein.

For example, an ion exchange along an external and an internal surfaceof the glass article may be used to form an asymmetric compressivestress layer. An example compressive stress layer for a glass articlecomprising a thicker peripheral portion and a thinner central portioncomprises a first compressive stress region having a first depth alongthe external surface in a central portion of the glass article and asecond compressive stress region having a second depth, less than thefirst depth, along the internal surface in the central portion of theglass article. The compressive layer further comprises a thirdcompressive stress region having a third depth, greater than the firstdepth, along the external surface in a peripheral portion of the glassarticle and a fourth compressive stress region having a fourth depth,less than the third depth, along the internal surface in the peripheralportion of the article.

In aspects of the disclosure, multiple ion exchanges are used to formthe compressive stress layer. The ion exchanges (alternately, ionexchange operations) may involve immersing the glass article in one ormore baths comprising an ion to be exchanged for a smaller ion in theglass article. For example, the glass article may comprise sodium ionsand the bath may comprise potassium ions as previously described.

Baths used in the multiple ion exchanges may differ in bath compositionand/or bath temperature. When it is desired to form a region of thecompressive stress layer having a relatively shallow depth and arelatively high compressive stress layer, the bath may have aconcentration of the ion to be exchanged which is greater than that ofother baths used in other ion exchange operations. In addition, theglass article may be immersed for a shorter time than used in other ionexchange operations.

The bath may comprise one or more salts including the ions to beintroduced into the glass; generally, the one or more salts at leastpartially dissociate into anionic and cationic components in the bath.In embodiments, the bath comprises a solution including the ions to beintroduced into the glass. In additional embodiments, the bath mayconsist essentially of the salt, so that the concentration of the saltin the bath is about 100%. The bath may be at a temperature where theone or more salts are molten.

In embodiments, the operation of forming a compressive stress layerhaving regions of different depths includes at least one operation ofapplying a mask to the glass article. In embodiments, masking techniquesmay be used to form each region of compressive stress separately, sothat the number of ion exchange operations is at least equal to thenumber of compressive stress regions. For example, to form a firstregion of compressive stress along a first surface of the glass article,a first mask may be applied to the other surfaces of the glass article,leaving the first surface unmasked. To form the second region ofcompressive stress along a second surface of the glass article, thefirst mask may be removed at least from the second surface of the glassarticle and a second mask applied to at least the first surface of theglass article, and so forth. In further embodiments, at least one regionof compressive stress is formed as a result of multiple ion exchangeoperations.

An example of a method including an operation of forming a compressivestress layer comprising the first, second, third and fourth compressivestress regions is described as follows. The method includes applying amask to shield the internal surfaces of the glass article and theexternal surface in the central portion of the glass article. Forexample, the mask may be applied to the central interior surface, thecentral exterior surface, and the peripheral interior surface. Afterapplying the mask, the method further includes performing a first ionexchange along the external surface of the peripheral portion of theglass article to create the third compressive stress region along theexternal surface of the peripheral portion.

After the first ion exchange, the example further comprises removing themask from the external surface of the central portion and performing asecond ion exchange along the external surface of the glass article tocreate the first compressive stress region along the external surface ofthe central portion and to increase a depth of the third compressivestress region along the external surface of the peripheral portion.After removing the mask from the internal surface of the glass article;the operation further comprises performing a third ion exchange alongthe internal surface and the external surface of the glass article.

In embodiments, the third ion exchange creates the second compressivestress region having the second depth and the fourth compressive stressregion having the fourth depth along the internal surface. The third ionexchange also increases a depth of the first compressive stress regionto the first depth greater than the second depth and the fourth depthand increases the depth of the third compressive stress region to thethird depth greater than the first depth. The second depth and thefourth depth may be about the same.

In additional embodiments, the mask is a first mask and the methodfurther comprises applying a second mask to the external surface of thecentral portion and to the external surface of the peripheral portionprior to performing the third ion exchange. The third ion exchangecreates the second compressive stress region having the second depth andthe fourth compressive stress region having the fourth depth along theinternal surface. If the second mask substantially blocks ion exchangeof the external surface of the central portion and to the externalsurface of the peripheral portion, the second ion exchange creates thefirst compressive stress region having a first depth and increases adepth of the third compressive stress region to the third depth.

As used herein, the terms “about”, “approximately,” and “substantiallyequal to” are used to account for relatively small variations, such as avariation of +/−10%, +/−5%, or +/−2%.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the describedembodiments. However, it will be apparent to one skilled in the art thatthe specific details are not required in order to practice the describedembodiments. Thus, the foregoing descriptions of the specificembodiments described herein are presented for purposes of illustrationand description. They are not intended to be exhaustive or to limit theembodiments to the precise forms disclosed. It will be apparent to oneof ordinary skill in the art that many modifications and variations arepossible in view of the above teachings.

What is claimed is:
 1. A glass article for an electronic devicecomprising: a central zone having a first thickness and comprising: acentral exterior surface and a central interior surface; a firstcompressive stress region extending from the central exterior surface toa first depth; and a second compressive stress region extending from thecentral interior surface to a second depth; and a peripheral zone atleast partially surrounding the central zone and having a secondthickness that is greater than the first thickness, the peripheral zonecomprising: a peripheral exterior surface and a peripheral interiorsurface; a third compressive stress region extending from the peripheralexterior surface to a third depth that is greater than the first depth;and a fourth compressive stress region extending from the peripheralinterior surface to a fourth depth that is less than the third depth. 2.The glass article of claim 1, wherein the second depth is less than thefirst depth.
 3. The glass article of claim 1, wherein a compressivesurface stress of the fourth compressive stress region is greater thanor equal to a compressive surface stress of the third compressive stressregion.
 4. The glass article of claim 1, wherein: the second thicknessof the peripheral zone is less than 1 mm; the peripheral zone furthercomprises an internal tensile stress region; and a thickness of theinternal tensile stress region is at least 20% of the second thickness.5. The glass article of claim 1, wherein: the peripheral exteriorsurface comprises a curved region; and the curved region of theperipheral exterior surface adjoins the peripheral interior surface. 6.The glass article of claim 1, wherein: the peripheral exterior surfacecomprises a curved region; the peripheral zone of the glass articlecomprises a side surface; and the curved region of the peripheralexterior surface adjoins the side surface.
 7. The glass article of claim6, wherein: the third compressive stress region extends from the curvedregion of the peripheral exterior surface to the third depth; theperipheral zone further comprises a fifth compressive stress regionextending from the side surface to a fifth depth; and the fifth depth issubstantially equal to the third depth.
 8. A glass article for anelectronic device, the glass article comprising: a first portion havinga first thickness and comprising: a first front surface and a first rearsurface; a first compressive stress region having a first depth alongthe first front surface; and a second compressive stress region having asecond depth, less than the first depth, along the first rear surface;and a second portion contiguous with the first portion, having a secondthickness greater than the first thickness, and comprising: a secondfront surface and a second rear surface; a third compressive stressregion having a third depth along the second front surface; and a fourthcompressive stress region having a fourth depth along the second rearsurface, at least one of the third depth or the fourth depth beinggreater than the first depth.
 9. The glass article of claim 8, whereinthe glass article defines: a central zone including the first portionand the second portion; and a peripheral zone at least partiallysurrounding the central zone, having a third thickness greater than thefirst thickness, and comprising: a third front surface and a third rearsurface; a fifth compressive stress region having a fifth depth alongthe third front surface; and a sixth compressive stress region having asixth depth along the third rear surface, at least one of the fifthdepth or the sixth depth being greater than the first depth.
 10. Theglass article of claim 9, wherein: the second portion further comprisesa first wall surface between the second rear surface and the first rearsurface; and the peripheral zone further comprises a second wall surfacebetween the third rear surface and the first rear surface.
 11. The glassarticle of claim 10, wherein the third thickness is greater than thesecond thickness.
 12. The glass article of claim 11, wherein: the glassarticle defines a length and a width; the second portion defines a ribfeature; and the rib feature extends along the length or the width ofthe glass article.
 13. The glass article of claim 9, wherein: the thirddepth and the fifth depth are each substantially equal to the firstdepth; and the fourth depth and the sixth depth are each greater thanthe first depth.
 14. The glass article of claim 9, wherein: the thirddepth is substantially equal to the first depth; the fifth depth isgreater than the first depth; and the second depth, the fourth depth andthe sixth depth are each less than the first depth.
 15. A method formanufacturing a glass article, comprising: forming a compressive stresslayer through at least one ion exchange along a central portion and aperipheral portion of the glass article, the peripheral portion having athickness greater than a thickness of the central portion, and thecompressive stress layer comprising: a first compressive stress regionhaving a first depth along a central exterior surface of the glassarticle; a second compressive stress region having a second depth alonga central interior surface of the glass article; a third compressivestress region having a third depth, greater than the first depth, alonga peripheral exterior surface of the glass article; and a fourthcompressive stress region having a fourth depth, less than the thirddepth, along a peripheral interior surface of the glass article; therebyproducing a region of a tensile stress within the glass article tobalance the compressive stress layer.
 16. The method of claim 15,wherein the fourth compressive stress region is formed after the firstcompressive stress region and after the third compressive stress region.17. The method of claim 15, wherein a compressive surface stress of thefourth compressive stress region is greater than or equal to acompressive surface stress of the third compressive stress region. 18.The method of claim 15, wherein the at least one ion exchange comprisesat least three ion exchanges.
 19. The method of claim 15, wherein thefourth depth is substantially equal to the second depth.
 20. The methodof claim 19, wherein the operation of forming the compressive stresslayer through the at least one ion exchange comprises: applying a maskto the central interior surface, the central exterior surface, and theperipheral interior surface; after applying the mask, performing a firstion exchange along the peripheral exterior surface to create the thirdcompressive stress region along the peripheral exterior surface;removing the mask from the central exterior surface; performing a secondion exchange along the central exterior surface and the peripheralexterior surface, thereby: creating the first compressive stress regionalong the central exterior surface; and increasing a depth of the thirdcompressive stress region along the peripheral exterior surface;removing the mask from the central interior surface and the peripheralinterior surface; and performing a third ion exchange along the centralinterior surface, the peripheral interior surface, the central exteriorsurface and the peripheral exterior surface of the glass article,thereby: creating the second compressive stress region having the seconddepth along the central interior surface and the fourth compressivestress region having the fourth depth along the peripheral interiorsurface; increasing a depth of the first compressive stress region tothe first depth greater than the second depth and the fourth depth; andincreasing the depth of the third compressive stress region to the thirddepth greater than the first depth.